My brother failed to get a new place to live when he was told 90 days ago that he had to move from his rental. Now he is moving in with my mom, my daughter, and myself. She made this decision while my daughter and I were gone. I do not get along with my brother and am very hurt that I was not even considered before she did this.
Looking at it solely from your mother's point of view: when her daughter was at a low ebb, she offered her a home. Now her son's in a tight spot, she's offered him a home.
She doesn't need your permission to do this, you know. And if the decision was made, as you put it, while you and your daughter were away, then it can't also be the case that you're constantly at her side, can it.
How long do you think your brother will want to stay?