That's really tough. My instinct would be to write your sister off. But I understand that you don't want ot lose contact with her children. How old are they? Is there any way to have some sort of contact with them without your sister's involvement? Or, would that be posible in a few more years? I guess in the meantime, just do limited visits to maintain some level of contact. If she starts violating your boundaries, then I'd cut the visit short.