I think all the (serious) considerations mentioned so far are secondary to the fact that if her mother finds out there will be a homeless sixteen-year-old girl on the street. I certainly wouldn’t want to have that on my conscience and I doubt you would either. Mom sounds like a crackpot and the best thing you can do for this girl is to stop seeing her now.
Sure, it’s possible that the girl might be exaggerating, or that her mother might not go through with it. But since you don’t know, you may well be consigning this girl to a life of selling herself on the street for food and motel money…and the drugs that usually go with it. The possible consequences are simply too dire to gamble that she’s exaggerating or that her mom won’t go through with it.
Slight epilogue: she told her mom she wanted to be with me and now I’m not allowed to see her any more. I went to her house to try to say goodbye, and her mom told me “Oh, you’re fine, you’ve done nothing wrong. It’s just that she isn’t ready to have a boyfriend; she has developmental disorders, and she isn’t 15 like she told you”.
…She told me she was 16… I’m not even going to think too hard about that.
Yes, or at very least just stick with the grannies, as that will probably not land you in prison. The same can not at all be said for a pattern of “dating” troubled, underage girls when you’re a legal adult, as well as hanging out with troubled girls as young as 12.