What would happen to my brother and me if my mom died? (According to law)?

Likely, your dad would have custody of you. He could make you live wherever he wanted to-your house, his house, or somewhere else. At 13 and 14, you could not live by yourselves. Possibly, if your father is abusive or otherwise unfit, your mom could designate someone else as your guardian if she died. You'd have to see a lawyer about that.