I'm siding with you as well. No one has the right to call your brother a dumbass but you. And your mother shouldn't be getting mad at you about YOUR relationship.
I'm siding with you. Your mom was being pretty stupid when she said that you were being overly sensitive. In most cases, the mom would probably have went and kicked the boyfriend's ass for saying that her son was a dumbass (I know I would at least).
I'm siding with you as well. I'm super close to my sister. I'll stick up for her no matter what. Even if she doesn't love her brother like I love her, I'll never let anyone but me insult her.