It's not her inheritance at all. It's just a gift you promised her. After what she did, you would have every right to withdraw that promise. There was no reason for her to think that that promise was in no way conditional on her behavior. I don't know what she did to her little sister, but it sounds as though it must have been pretty awful. Just remember, though, that making this decision could permanently impact your relationship with her.
It's not her inheritance at all. It's just a gift you promised her. After what she did, you would have every right to withdraw that promise. There was no reason for her to think that that promise was in no way conditional on her behavior. I don't know what she did to her little sister, but it sounds as though it must have been pretty awful. Just remember, though, that making this decision could permanently impact your relationship with her.