Hey,
Your family has been though a lot. I get why your mum is scared, the abuse from your dad must be hard for her to deal with, your sisters behaviour must have been hard for her to deal with.
I get that you love your mum, I love my grandad. But there must be some part of you that resents her for not sticking up for you no matter how hard it was for her. She's your mum, she is supposed to protect you. (Apparently, I never got protection from mine, but that's what is supposed to happen)
Well, it can't be ignored. It must have been hard as a child to understand why she was allowed to get away with it and why you should have to ignore it.
Feeling guilty in hindsight is hard. You were a kid, getting beaten up, and taking emotional beatings too. You said what would hurt her out of fear and anger. It is completely understandable, and you need to find a way to forgive yourself. You will only drive yourself mad in the long run. What you said can be forgiven considering the circumstances. This might be hard, but imagine two kids you know. Imagine that one was behaving like your sister and one retaliated like you. Now look at them, would you blame the kid for retaliating? I know looking in that I wouldnt.
"I guess that I still feel badly because, even though at the time I really didn't understand what was wrong, I still wasn't sensitive enough to her feelings and needs. And I know that this makes no logical sense for me to feel this way; at the time all of this was going on, I wasn't even taking care of myself or my needs and feelings. I know that, while I don't logically owe her anything, that I still feel like she owns me in a sense."
It was never your responsibilty to be sensitive to her needs and feelings. This is something you should mention to your T if you have one. The logical vs illogical is from what I remember from my ex T is that it comes from the contradiction in thinking as a child an an adult. But you should listen to the logical whenever the illogical takes place because you owe your sister nothing, she owes you. She doesn't own you, she never has and never will.

Yes it's great that she has mellowed with age and being a mum.
It is completely normal believe it or not to start expressing your anger. The more you bottle the stronger it becomes. Look at it this way, if you shake a soda can really hard and then even just pull the top a little a ton and a half of fizz comes out. Sometimes you start to wonder if the can had doubled in size, and gets really messy.
I have had a lot of anger issues. My ex T said I had every right to feel as angry as I did/do just got to find the right release for it. They are YOUR emotions, you need to get them out. I hope you do have a T, and you can discuss this all with them. They can really help.
Obviously I am no therapist. (Although sometimes I think my friends think I am a free one. I don't really mind but sometimes their minuscule dramas bore me.)
You really need to start thinking about you. It is not selfish, it's the only way to get through these feelings and emotions.
Your family will always be your family even if they could keep Jeremy Kyle (or an American equivilant like Jerry springer) busy for months on end! And you will always love them.

I hope what I have said makes sense.
Hang in there
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THERE IS NO SUCH THING AS NORMAL... JUST DIFFERENT LEVELS OF MESSED UP!