The Long Lost Black Princess

After a one night stand with Lily Potter, Sirius Black finds out he has a daughter. He's always wanted a little girl, but when Lily dies he doesn't get her like he's supposed too. Once Ginerva gets to School she not only learns that she's a black, but that she's also a vampire. What happens when there's a potions accident and including being aged, she winds up soul bound to Snape.