Hi,
Buffy and spike are brother and sister and sleep together but one day Joyce walks in on them.
Spike leaves but comes back after a long while(maybe years- I'm not sure)
Buffy is married
(can't remember his name)
and they go back home for Christmas or some kind of holiday and Buffy and spike start sleeping together again.

At one point-
Spike is a photographer and shows his photos with everyone at one point Buffy and spike are annoying each other and Buffy's husband walks in on them and is uncomfortable at they way they behave around eachother
.
At the end Buffy and spike live together somewhere and Buffy paints or draws.

Hi,
Could be The Bond by Grief Counseling (Dampersandspoons).
It's the only story I know of, in wich Spike and Buffy are siblings (twins to be precise) and not step-siblings.
Hope that's the one, let us know.

Hannora
Hi,
Yes that is it thank you,
By the way the link sent me to another fic called 'THE COLOR OF DEVOTION'.

Buffy_newbie2020
Sorry about the mistake, it's fixed.
That's what you end up doing when you try multitasking with a foggy brain.