I would just take myself right out of this. This is an issue between your mother and brother. When you catch yourself getting upset about this, just remind yourself that it is not your business. Your brothers are old enough to stick up for themselves, and so are the girls they are dating.

Kaliope is right that this is fairly common, just speaking as someone who has dated guys with mothers like this

If your mother is treating *you* like you are not worthy of female attention i.e. saying something rude to you along those lines, it's totally your business. I would just leave the way she acts about your brothers and their girlfriends alone.