It sounds like your sister in law has mastered her own form of grey rocking. She just ignores it. That is being a grey rock.

Being a grey rock is all about not taking on someone else's poor behavior. After all a rock doesn't care how bad someone's behavior is.

I took a workshop years ago, and learned to create a big beautiful rose in my mind, and placing that rose around me. It acts like a barrier and to me is more warm, gentle and beautiful than being a rock. You could look online at pictures of roses (or a rose bush) and pick one out that resonates with you if you can't picture a rose. Sounds silly, but it is what I had to do. I still use this method today.

Your brother is a deeply troubled man, and his mean behavior has to do with him, not you. I repeat, his awful behavior is all about him. He's just projecting it out onto you. Allow the rose to absorb it.

Another similar method I learned at the same workshop is hold or place an imaginary rose out in front of you. Imagine all his negative energy going into the rose instead of you. Afterwards imagine throwing that rose into the air and watching it blow up into a million transformed pieces and flowing down into blessings on you.

If a grey rock or rose isn't to your taste, you could try a lotus flower or chrysanthemum.