Op! Come back and give us more details because your post was confusing.

I don't understand why Mom is booking and paying for all of these adult children's plane tickets. And now these "children" are fighting over Mom playing favorites!

Amazing. I'm sure this will be one he** of a trip. Good luck.

My MIL, if her daughter, my SIL, had kids, wouldn't even think about ours. A mother is going to bond differently with her own daughter becoming a mother than a DIL. Would the DIL even be grateful and does she want a more intense relationship than what she has? Grandparents aren't just free help...when my mom moved in with us, she encountered a series of health issues, we needed a special vehicle for her and she had been conned out of her savings by a predatory spouse (not my dad). I used up all of my sick leave caring for her and have none for my maternity leave. Just saying...maybe OP's parents don't trust the brother and his wife to return their love when they are less capable.

Btw, my mom highly favors my brother. So I feel OP's brother's pain, but there's just nothing you can do about it.