I just wanted to add the story here; it may be not what you're looking for. Because it's a WIP and it doesn't seem as if there will be an update soon (see the post in her LJ; RL happens). But I haven't given up yet so cross your fingers.
Long Time Gone by bitterness
It still says:
Status: Currently being re-written. Thirty-six chapters posted. Looking at 40+.
AU all-human. When Buffy and Spike were in high school, everything was nearly perfect for them-- until she mysteriously disappeared. Now, five years later, Buffy's returned for her mother's funeral, and to take over caring for her sister, Dawn. Can Dawn and Spike ever forgive her for running away? Especially when she won't tell them why? Genre: AU, angst, and sorta baby-fic Warning: deals with rape.
Oz is Spike's friend and together with Willow; later he falls for Dawn (Willow left because of Tara). Sadly it's WIP.

Still worth a read.