Talk to your parents about it. Tell them you didn't like your Christmas gift, and don't let your brother boss you around. Your parents can, your brother can't. Your parents gave you life, sorry to tell you but they can do anything they want if it's legal. But your brother can't. Let your parents know that you shouldn't have gotten in trouble for the stuff he doesn't. Tell them he makes you feel really awful and you don't like it. If they're good people they'll understand.
I would stop being friends with her. I mean she should understand that it'd be mean to do that. It's hard to help your friend get through a breakup when you're dating the same person. Plus you don't even know if she was trying to break you guys up so she could be with him.
Talk to your friend. Tell her that this is the first guy that's ever liked you but you don't want her to get mad at you for stealing him from her. But understand she likes him too so don't get mad if she doesn't want you dating him. Nicely decide which of you should get him instead of the other. Chances are she might be thinking the same thing.