Re: Bullied by sister parents turn blind eye
I don't think there's anything you can do. Nor should you, as it's his family issue to deal with and not yours. People who get overly involved with their spouses family drama usually end up in a lot of hot water and it only causes more problems than it solves.
As he is an adult now, he has the choice to be around the family or not. It's up to him. If his sister hasn't matured past 12 and wants to keep being an idiot for the rest of her life, he doesn't need to be around her anymore. People have cut off family for less than this, it wouldn't be unusual for him to cut her off. I think he just needs to stop allowing her in his life to cause stress. He needs to stay away from her and if she ever comes crawling back to him, he should just say, Sorry, I'm done with you.