I appreciate these tend to be human fics and they tend to be 'kind of' related. The ones I've come across are below, are there any others anyone can add?

Cheers

Crave by nautibitz (father-in-law) - WIP although it does get as far as epilogue type chapters
Older by nautibitz (stepfather)
Guardian by Pyro63 (guardian)
Sweet dreams by paganbaby (stepmum)
Been Here Too Few Years by Addie Logan (step-brother)
Deny Thy Father by Vanilla (stepmum)
Silently Broken by suzee (stepfather's brother)
Oedipus Calling by Addie Logan (stepmum)

The Hunters by greyangel (brother in law) - not online anymore as far as I know but i do have a copy if you want it.

Constant Craving by OKDeanna (William is the godson of Buffy's stepfather, so kind of related)

Family In Law by shampoocommercialhair (brother in law)

Little by Little by facingthesun (Spike is Joyce's fiancee)

The Trouble with Keeping Promises by Jaesha (uncle by marriage)

The Bond by dampersandspoons
Fraternal twins Buffy and William have a special connection that others are starting to grow suspicious of.  Unaware that their behavior is wrong, Buffy and William become closer than they imagined possible.  But will their secret remain hidden or will they have to face the consequences that come with the choices they make?

http://dark-solace.org/elysian/viewstory.php?sid=2675&warning=Adult%20Only


The Bond, Continued by dampersandspoons
Summary:
*Sequel to The Bond.  Yes, you will need to read it first.*

It is eight years later for fraternal twins Buffy and William.  How much has changed for them over the years?  What secrets have they kept hidden?  What consequences were a result of their actions?  And, can it ever be repaired?


http://dark-solace.org/elysian/viewstory.php?sid=2679&warning=Adult%20Only

Also

The Thin Red Line by dampersandspoons
Summary:
Step-siblings Spike Pratt and Buffy Summers are close to crossing the line. Ever since they first met nearly four years ago, they've had to fight the physical attraction between them, fight to ignore the lust that taints their blood, the low moaning and hoarse shouts of self-gratification that slip through the thin walls of their adjoining bedrooms each night...  (Written for the taboo_spuffy 002 Mini Challenge)

http://dark-solace.org/elysian/viewstory.php?sid=2431&warning=NC-17

And
Protect Me From What I Want by Rae

Summary: Buffy and Spike are stepsiblings. When Spike started seeing Dru and then eventually went away to live with her, him and Buffy drifted apart (No actual Dru/Spike scenes planned). Buffy went away to college and made a new life for herself, then one day out of the blue Spike rings her asking for a place to stay. After years of estrangement they realise they no longer know how to act like brother and sister. While they're getting closer again they find out how dangerous that can be.


http://spikeluver.com/SpuffyRealm/viewstory.php?sid=34016&warning=AO-AdultsOnly

Protect Me From What I want has a sequel.

Protège Moi by Rae

http://spikeluver.com/SpuffyRealm/viewstory.php?sid=37201

Summary: Sequel to 'Protect Me From What I Want' (which can also be found on this site)...Buffy and Spike's parents married when they were young and the two grew up as siblings, however through adolescence they drifted apart, and when circumstances reunited them they both started to battle strange feelings for one another, which eventually ended in them falling in love...picks up right where the first story left off, after months together they've finally reconciled their problems and the struggle they initially had with their relationship, and decide to go home to Sunnydale to tell their family...however when they return tragedy strikes causing them to postpone their announcement...will it bring them closer or threaten everything they've worked so hard to build? Complete

Spike is Buffy's step dad in this.

Window to the Soul by Starlight_Slayer

http://spikeluver.com/SpuffyRealm/viewstory.php?sid=37166&warning=NC-17

Summary: Her love for him was forbidden...a childish dream in a world of harsh realities. But when tragedy strikes, can Spike and Buffy overcome the barriers to their love and pull closer together, or will the real world tear them apart? Complete

Six too Many by TammyAsh666

http://spikeluver.com/SpuffyRealm/viewstory.php?sid=20598

Summary: Two completely different families are forced to live together when their parents get married. They learn to adjust to the new arrangement and find out that they might be more alike than they think. Spike finds himself drawn to his new step-sister, but will she feel the same way about him? What consequences would they have to deal with? Insider Tip runner up at the Cradle of Humanity Awards. Complete

Sweet Surrender by Sophie

http://spikeluver.com/SpuffyRealm/viewstory.php?sid=823

Summary: COMPLETED ! Spike's mom, Joyce, married Buffy's dad, Giles, twelve years ago. They were raised like brother and sister. Spike has became a famous TV star and is engaged. Buffy stayed in Sunnydale where she works at the high school and dates Riley. Buffy's feeling for Spike were never the sisterly kind, but she bound them, thinking he could never love her back like she wanted him too. Unless... Complete

This is forever WIP.
It doesn't even end in a good place but it's already really long and has Spuffy.

The Attic by jennybean

http://spikeluver.com/SpuffyRealm/viewstory.php?sid=14468

Summary: After the death of her husband, Joyce is forced to move with her four children back to Sunnydale to live with the parents that disowned her years ago. At Rayne Manor, siblings William and Elizabeth try to protect their family from their evil grandmother Drusilla, while uncovering secrets of the past and creating a few of their own. Loosely based on the book 'Flowers in the Attic.' WARNING: Will eventually deal with incest. WIP

Touch Myself by blatravis

http://spikeluver.com/SpuffyRealm/viewstory.php?sid=24247

Summary: What's a guy to do, when he catches his step-sister doing something naughty? This is my first fic, so please be nice This fic deals with incest, so if you dont like that kind, then please dont read. Complete

Summer Skin by Vanilla

http://dark-solace.org/elysian/viewstory.php?sid=2449&ageconsent=ok

Summary:
Step-siblings Buffy and Spike have worked very hard not to give in to the wild passion flaring between them over the years, but now that they're both single, the dangers of them actually giving in to what they want will rise...Written for the taboo_spuffy community mini challenge on LiveJournal. Complete

Dancing with fate. by Spikes_slayer08

http://dark-solace.org/elysian/viewstory.php?sid=3269&warning=NC-17

Summary: AU all human fic. WARNING: this is a half sibling fic (same dad) if that's not for you then go no further otherwise enjoy. Buffy and Spike are half siblings who have the same father but Spike has spend the past two years fighting his attraction to Buffy and with only two weeks left before he leaves for college he thinks he is home free but when their parents decided to go away on a second honeymoon Spike finds himself alone for the full two weeks with the object of his every desire. Complete

Little Sister Don't You by Brat (Step-siblings)Summary: **Nominated at VK Awards for Best Plot and Best Angst and me for Best Author!!**Spike and Buffy are step siblings. Spike left home when he was nineteen and Buffy was just sixteen. Four years have passed and Spike has come back home. He has a secret--what is it and will Buffy understand when she finds out what it is? **See disclaimer about this idea on Chapter 1 Authors Notes**

http://spikeluver.com/SpuffyRealm/viewstory.php?sid=13604&warning=NC-17

It has a sequel that you can access on Brat's page.

Love takes time by Brat (Step-Father)Summary: Buffy has returned to Sunnydale to bury her mother after having been estranged from her for the past five years. The cause of the problem was her mother's marriage to a certain younger man named Spike. Having to face one another for the first time in five years, Spike and Buffy find that they might need each other more than they realize. Winner at Love's Last Glimpse awards for Best Fantasy! Nominated at Spark and Burn Awards for Best Fantasy, Romance and Best Spike Characerization. Thank you!!

http://spikeluver.com/SpuffyRealm/viewstory.php?sid=23720

In The Shadows With Me by Megan_Tam (Uncle) Summary: Sometimes even the brightest light can't penetrate the darkness.
After years of unbelievable abuse, a broken and battered Buffy is finally rescued by her Knight in Shining Armor, but will their unique relationship and his boundless love be enough to save her? WIP

http://www.sweet-temptation.net/viewuser.php?uid=64

Is this the sequel? Her site doesn't work even on wayback now.

A Little Sister, Don't You Christmas by Brat

http://spikeluver.com/SpuffyRealm/viewstory.php?sid=16264

Summary: Spike's got a surprise for Buffy on their first Christmas together and he's got an audience for it too!

She also has another step sibling fic.

Starting Over by Brat

http://spikeluver.com/SpuffyRealm/viewstory.php?sid=21923

Summary: Spike and Buffy are complete opposites, but they are stepsiblings. They didn't grow up together, but they were close once upon a time. Buffy has just blown into Spike's life after a long separation...and Spike is finding his feelings haven't changed all that much. Complete

I found this one.

Hey Little Sister by slayershandbook

http://spikeluver.com/SpuffyRealm/viewstory.php?sid=33279

Summary: Stepsiblings Spike Pratt and Buffy Summers are close to crossing the line... Complete

Spuffygirl --- yes that is the sequel. I'm sorry I meant to say her author page. I was being lazy about linking things.

And good catch on the other one. I forgot about that one

Thanks.

More fics.

Merry Christmas by holetoledo

http://spikeluver.com/SpuffyRealm/viewstory.php?sid=33254&warning=AO-AdultsOnly

Summary: Buffy and Spike are step-siblings who can barely stand one another, but it's the Holidays, and after - and even before - a cup of cheer, well... You can find out for yourself.

Growing Up by TammyAsh666

http://spikeluver.com/SpuffyRealm/viewstory.php?sid=12953&warning=NC-17

Summary: Buffy and Spike practically grew up together. Their parents are married and they have always had nothing but a sibling love for one another. What will happen when they start to see each other differently? Warning: A relationship between step-siblings!

This one has them growing up together as family.

Carry Me Away by TammyAsh666

http://spikeluver.com/SpuffyRealm/viewstory.php?sid=27353

Summary: Buffy witnessed a tragedy when she was a little girl. She's taken in by the Giles family and forms a close bond with their son, William, but things change as she gets older. Can Buffy ever be the happy child she once was? A story of loss and finding love where you least expect it.

Dangerous by TammyAsh666

http://spikeluver.com/SpuffyRealm/viewstory.php?sid=32254

Summary: Spike was married to Joyce, but she passes away and he becomes responsible for her two teenage daughters. Buffy is tired of always being treated like a kid, so she's very determined to get him to finally notice her. Runner up of the Respected Family Award at Cradle of Humanity.

Forbidden Fruit by TammyAsh666

http://spikeluver.com/SpuffyRealm/viewstory.php?sid=36358

Summary: Spike is trapped in a loveless marriage and can't stop thinking about his step-daughter, even though he knows she's off limits. To get his mind off of his wife and the girl he can't have, he takes his friend's advice to call an escort service for one night, but wasn't expecting who decided to show up.

Love is Blind by TammyAsh666

http://spikeluver.com/SpuffyRealm/viewstory.php?sid=36725

Summary: After her mother's death, Buffy is left alone with her little sister, but the fact she was born blind makes her an unfit guardian. Her only hope is Spike, her recently divorced next door neighbor, who will do whatever he can to make sure the sisters don't get separated, even if it means taking them in his custody.

Learning to Breathe by TammyAsh666

http://spikeluver.com/SpuffyRealm/viewstory.php?sid=21760

Summary: William is married to a woman named Elizabeth and expecting his first child, but there were complications and he ends up having to raise the baby on his own. He puts out an ad for a nanny and when Buffy shows up to apply for the job, he gets the shock of his life. Nominated at the Love's Last Glimpse Awards, Lost in Spike Awards, and Cradle of Humanity Awards.

Wow!! I'm gonna be busy! Thanks everyone.

I'd love The Hunters SMac, do you still have my email? I keep meaning to check which greyangel fics people have as they are always good.

No, I clean out my email constantly. PM me and give it to me again and I'll send it to you.

Blood Oranges and Sequel by Minx Delovely her on SR. Dru and Angel are brother/sister and respectively married to Spike and Buffy. Marital issues and tragedy move Buffy and Spike together.

Guest
Hey can I get blood oranges and the sequel I loved it. I'm at *********** thanks.

Edited by mod because email addresses are not permitted in the threads. If you wish to have a fic exchange made please PM the person who offered and make the transaction off the thread. Click the envelope next to the person who offered's name and that will take you to where you can PM them. If you click on their actual name you will go to their author page where you can directly email through their contact link.

Thank you.