Hi mlewis! Sounds like you have a lot on your hands right now. I was drawn in to your story because a lot of the beginning of it sounded a lot like my abf's life growing up. I know how easy it is to "understand" why they are the way they are based on how they grew up. But, I also know now that it's not okay for it to affect my life in such a negative fashion. It's NOT OKAY that he is abusing you. You DO NOT have to stay in this situation and in my opinion, you SHOULD NOT stay in this situation. You have the weight of the world on your shoulders trying to make everything right. Trying to be the good "mom" and protect your bf and his siblings, but it's not your responsibility.
I think if I were you, I would place a call to social services. These children need to be protected and need stability. I'm so sorry that you're going through all of this. Also, if you can get yourself to alanon meetings and/or maybe to a counselor for support, that would be a step in the right direction.
Hang in there and don't forget to take care of your self!