Please read! Much needed advice!

I know this is long but please read, I'm desperate for advice. I need advice on a family issue. My husbands brother and sister are estranged from my husband. About 5 years ago, my husbands sister accused my husband of inappropriately touching her when they were younger. She says she was three or four meaning he was about seven or eight. When she said this to the family, it hurt everyone. My husbands brother got angry and will not talk about it. My husband says he never did it. He is a wonderful guy and I believe him. Not just because he is my husband but also because he's not a liar and he is the best man I know. We have been married for five years now. In October, we had our first child, a beautiful girl. When I was pregnant, I tried to communicate with both siblings to work on resolving issues and they ignored me. Their mother got cancer when I was about 7 months pregnant. When this happened they still wouldn't get together. I have been doing everything I can think of to get them to say anything to me. I talked to their mother and father and they are giving up. Their mother is enabling the situation by purposely keeping everyone separate. She will schedule different holidays to make sure no one is at the house at the same time. I have written kind letters to everyone trying to some response. I have messaged them on Facebook and they have blocked me. I never did anything to them. Why are they so angry. I just want to talk with them and hear their side of things. They have a niece now and won't even come together to meet her. My husband has tried everything. A while back, the brother and sister said they would give it a chance if my husband saw a therapist. He did and the therapist said after one session that there is nothing to be done because my husband didn't do it in his eyes. Their mother keeps pretending to the world that nothing is wrong. She wants to keep seeing my daughter and act like everything is fine. I have reached the point where I don't want to see my husbands parents anymore because I have no respect for them because they won't fight for their family and I don't want them to see my daughter either. I don't want to give up on the family. I don't really give up on anything. But this is unhealthy and as my daughter gets older, it will be unhealthy for her too. Any advice?