How do you know your boyfriend's sister says bad stuff behind your back? Or do you mean she actually stands behind you and says bad things? I think you have something from her brother she will never have. Take the high road and ignore her nasty remarks and see if they taper off. If they don't you should sit down with your boyfriend and tell him his sister is annoying you and you don't intend to stand for it and he can either man up and stand up for you or you will stand up for yourself. Keep in mind that if you are not married to this man, these people aren't your in-laws, so you don't really owe any 'family loyalty' to a woman who isn't your family and who has a poor way of showing her love for her brother by abusing his girlfriend.