I think my brothers pregnant girlfriend is being manipulative by not allowing their baby to have my brothers last name until they are married
Why do you believe that that's manipulation?
I feel that she's just in it for the money with him, you know?
It's obvious that you care about your brother, but you shouldn't insert yourself into his relationship. It's up to your brother and his girlfriend what they name their child, and by inserting yourself into the situation you're violating their privacy. Mind your own business, otherwise your brother and his girlfriend are likely to distance themselves from you.
But I can't stand that bitch controlling my brother.
Whatever you may feel about her, don't insert yourself in their relationship deeper than you already have. It's rude to do otherwise.