Chapter 19: Buffy/Charmed
Disclaimer: BTVS = Joss Whedon and Charmed was created by writer Constance M. Burge and was produced Spelling Television company, with writer-director Brad Kern as showrunner.
Fandom:BTVS and Charmed
Pairing: Chris/OC and Wyatt/OC (2-4)
Time-line: Season 6 of Charmed and before Buffy is 18.
Buffy is Leo Wyatt's Granddaughter. His wife had Joyce after he died during WWII [Elders hid Joyce from him]. Joyce dies and Leo is given guardianship of Buffy as her only relative.
I picked S6 because of the potential Buffy and Chris interactions and Buffy trusting him a lot more than the sisters did. Plus in the bad future Buffy could have cared for Chris after Piper died.