Well, Buffy and Spike were the only ones that got it on which isn't really stepcest but she made it kinkier by pretending they were stepbrother and sister. Giles and Joyce actually aren't in the fic (they're only on the phone in this fic) Here it is: http://community.livejournal.com/lovebite_fics/20992.html

There's a few minor errors, English isn't the author's first language. But I thought it was pretty hot.