Nicely done -- believable characterizations and a very atmospheric setting. I like the way you frame the entire situation in terms of what it means to be a Black; I can imagine that "Blackness" in all its incarnations is something that no one in the family ever forgets, even when they try to deny it.

she loves the idea of not being a Black even more, and she lets Ted take her to Hogsmeade.
Very believable.