Have you tried talking to her about it? Maybe bring it up gently, just say, "I feel there are some negative feelings in our relationship as sister-in-laws. I really want us to get along, so please tell me if I have done or said anything to upset you, so we can make this right." I had to go through basically the same thing with my husband's sister. If she won't try to fix things, ask to her at least be civil at family gatherings and kids birthday parties, for the sake of the family and kids. If you don't feel comfortable talking to her, maybe your husband would? He could let her know he feels she is being unreasonable and hurting your feelings. I hope I helped a little bit Good luck!