Dad's girlfriend hates me, what should I do?

My dad has started dating again finally !!! My brothers and I were so happy when dad told us he was dating again. When we first met her she was so nice and sweet she seemed like she was perfect for my dad. But when ever we saw her she was with dad. But one day she stopped by two of my older brothers just came home and my dad wasnt home yet. My brothers were upstairs and I let her come in to wait for my dad. I was trying to be nice and tried to start up a conversation but she started acting all snobby and rude. She got all serious and started calling me names that I would be beyond grounded for repeating and she threatend me saying that if I ever tried to come between her and my dad I couldnt even imaging what would happen. Of course being me I used my big mouth and stood up for my self saying she had no right to talk to me like that. She just slapped me across the face, hard. So hard that I started to bleed. I screamed in pain so loud my brothers sprinted down the stair. I lied to them and just said I waked my face off the coner of the counter while benging down. I wanted to call my oldest brother and tell him but I knew hed just rush home so I didnt. I want to tell my dad or brothers but dad’s so happy with her and im afraid of what eles she might do to me if I tell anyone. Any advice on what to do?