One line says it all. She feels you “stole” her dad? My guess is that she is a terribly hurt person who is lashing out because she doesn’t know any other way to get the attention from your dad she craves. Hurt people generally hurt other people. A starting point for you is to remember that.
It’s not true that you didn’t get along as kids because you were raised separately. You didn’t get along because both of you wanted what you thought the other person got. I’m guessing that as a young child, you wished you could be with your mom and siblings. It’s also likely that she thought you had a better life with your aunt than she had.
You’ve both had it hard. There is no “winner” in the contest of who had it harder. It sounds like neither one of you had the stability of a loving family that every child needs and deserves. Since she is violent and you have violent fantasies, I’m guessing that both of you grew up in situations where you either experienced or heard about violence. Kids do tend to learn what they live.
It’s a shame that the two of you can’t be allies in healing. You don’t need to be competitors. If your dad is a loving father, there is enough love to go around. There is absolutely no need for you to bicker with her. You can’t be your stepmom’s defender. You can’t make your sister be more mature. She’s still working out the pain of her childhood. If you think your sister is out of line, say it once, and only once. Then let your father and stepmom iron out their issues with her.
You could try to talk with her reasonably and rationally. You two were really in the same situation even though you were in different places in your early years. Neither of you had the family you wanted and needed. Together you could restart your relationship by realizing that you could be there for each other now as two people who uniquely understand what it was like. She may not be able to listen but it’s at least worth a try.
I do think that you, your sister, and your father and stepmom could benefit from some family therapy. There are a lot of hurt feelings ricocheting around your house. With the help and guidance of a licensed therapist, you could all learn new ways to support each other in healing so your futures can be better than you past.
I wish you well.