Scapegoating

About a year ago, when my life was in chaos and I was in the throws of a possible divorce, my brother came up to "help" me.  How he viewed his role here as helping, I'm not sure.  When my 2 year old cried, he called him a pussy.  He criticized everything I did.  Why were my children using the baby potty?  They should be fully potty trained by now!  Why is there no food in this house?  (I'm a vegetarian- I had asked him if I could pick up any meat for him, as the food I'm accustomed to is very different from his, and he had said that wouldn't be necessary.)  My kids are terribly bad behaved.  No, they're toddlers.  
It was a nightmare- I needed someone to talk to.  Could I talk to my mother?
"If I tell you, can you promise not to tell P?"  I knew if she told my brother, there would only be more dissension.  He would pick me apart for being "sensitive."  
"Of course, J, why would I tell him?"
The next time I saw her-
"Well, I hope you won't be mad at me for this, but I told him."
Why did she do it?  The more I look into narcissistic mothers and scapegoating, the more I understand- she needs to foster dissension between us.  It makes her bond with the Golden Child stronger.  They can be cruel to me and complain to each other of my sensitivity.  They bond over their dislike for me.