Husband and his sister are having issues.. kind of long

Ok, little back story. My husband and his sister have always had a love/hate relationship. When they love eachother, its all good, when they hate eachother, its not "hate" but its passive aggressive anger. He drinks, she does pills. He has had a very colorful past and hasnt been the best of people at times. She has been one of those people who says she wants to help but always uses it against him and holds stuff against him years after.

Since my husband has been with me, he has changed his life around. He works very hard and is responsible... something he never was before. We have a wonderful life together. He has a job he loves and we live in a beautiful place.

His sister is still on pills, only now they are "prescriptions". She tries to point out things she thinks will get a rise out of him. He doesnt even let it bother him anymore.. when she is in that place, he just doesnt speak to her.

The only problem is, one of her "friends" , who I personally do not care for, is staying with her. This woman intensifies the dysfunctionality between my husband and his sister. She plants seeds of distruction and then dissappears when there is fallout. She will be at his sisters for Christmas and we promised his mother we would be there as well. I will go, as always, and be as pleasent and sincere as I possibly can be because it is what I was taught to do. My husband does not want to go because he doesnt want to "be made to feel like I am trash once again and that my life is still what it used to be."

I understand where he is coming from, I really do, but I dont want him to regret not being with his sister, mother and nephews on Christmas.

I told him we dont have to stay, but we really should go.

Would it be bad if I called his mother and her husband and asked them if they wanted to go out for dinner with us on Christmas.. or meet up with us before they head back across the state, where they live?

I always try to make the Holidays for him Amazing, because I was lucky enough to have amazing holidays.

My memories are filled with happiness and joy, yet his holiday memories, from when he spends them with his family, are filled with anger and yelling. He loves holidays with my family and wants to just spend them with my side.

I dont want him to regret it. I suggested having both families together but he said that my family is nice and he doesnt want to subject them to the disfunction that is his sister.

Im just out of ideas.

If you have advice or a similar experience, please leave it. If you need more info, just ask.

Thank You