Oh, this money issue thingy. I have a lot of experiences.
But I learned "No." is a complete sentence. And I feel much happy and better to say it when I need to. Don't lend it if you are going to regret or mad in the future. It will be even worse if you will bring it up with your mom or blame her used your money to help your brother later in life. It worse than not lend her the money. If you decided to "lend" her that money, just think that it's a gift for her, not your brother. And don't even bother how she spends it.