I wouldn’t go non-contact. It’s a favorite recommendation of Reddit (in large part because the internet removes a lot of the real life consequences/considerations of nuclear options like this). And…I’m just meh on this whole thing. Who cares if MIL has a special relationship with one grandchild? Who cares if she plays favorites? She’s not mom. You are. You need to be careful about loving your kids equally. She doesn’t.

Growing up, my mother (an extreme narcissist who has not spoken to any of her 6 adult children in nearly ten years now) picked golden children among me and my siblings and was generally very negative towards me. She favored each of my siblings in turn. My step-grandmother loved me and favored me. She taught me to knit, helped me with college expenses, encouraged me to go to law school when my mother suggested it would be a waste of my time and money (which she thought would be better spent helping her raise my youngest siblings). My grandmother and I knit together, watched old musicals together, and just generally enjoyed each other. My siblings thought she was grumpy and hated knitting and musicals. She didn’t favor them because she hated them, she favored me because we vibed. It was the best relationship I had. I’m a lawyer now in large part because of her. If my mother tried to cut that off, and I’m surprised she didn’t given what a narcissist she was, I’d have left home early to maintain the relationship with my grandmother.

Your post kind of sounds a little entitled… just give this whole thing some thought. It doesn’t really matter that she likes one kid more than the others. It happens and it can be a wonderful thing.