That's horrible greg
I kinda agree with Claudia, if she's an alcoholic it won't be long until she turns on your brother and he see's her for what she is, and you should tell your parents that too, because one day he'll end up coming back apologetic and they might react badly(?) If you get what I mean
I think with your friend you should just ask him outright, and if she does have a problem with you, you should tell him that you've known him a hell of a lot longer than he's known her, it'd be horrible for him to throwaway pretty much a lifetime friendship for a girl. Fair enough if she does has a problem but it shouldn't stop you from seeing your best friend.