I'm assuming it is your husbands sister, of course..... I have two side that desperately want to answer this question. Ok, that's out. Now, I would talk to your hubby and let him know how you feel and that he needs to handle, if he doesn't be a woman/mother and tell her about what she is doing and now that it has affected your children your not going to stand for it. The family should respect you for your actions, if they don't, I'm sorry, you have a long hard family life ahead of you.
