Oldish fic where Spuffy find out they are related?

I remember reading it a few years ago and think it was a one shot/two chapters at most.

Buffy and Spike are already a couple in it.

For some reason Giles and Joyce are reading a book and find out Spike and Buffy are related...he's her great, great grandfather or uncle I think.

Joyce runs home and bursts in on them and tells them assuming they will stop the relationship but neither bother and stay together.

Thanks in advance to whoever knows it.

Kissin' cousins by maryperk

Edit.... Nope, that's not it.

I found it.

It's Called Family Ties by Pamela