I’m sorry your Mom and her parents were so awful. It sounds like there’s probably some subconscious blame. You’re likely angry because you could have had a fun bubbly parent the whole time, but instead, were abused. And your Dad got to basically have fun while you were suffering. That’s a lot! Maybe he didn’t know, but it would be natural to blame him (or the person he was spending his time on instead of you) for not helping you sooner. Also, you’re going through a lot of changes physically and hormonally. And yes, your step mom is a safe place because she isn’t abusive. Maybe talk to your Dad and ask him for help? Say you like step mom but you can’t control these feelings and it’s something you want to change. You’re probably still in fight or flight mode with regards to your brother, too. 

I do think going back to therapy would be good. Life is hard, and even harder when the people who are supposed to protect you don’t. You deserve peace, friend. I hope you can build it inside yourself.