Here's my perspective on it from a similar situation.
I had to go no contact with my dad and stepmum, it was very much a last resort after decades of problems but one final incident that was the straw that broke the camels back, my stepsister however was lovely and nothing but nice to me, but in the end I cut the whole lot out of my life.
the root cause of these problems goes back to traumatic things that happened in my childhood that I had no control over and now as an adult I find that I compartmentalise things to protect myself, If I had kept in contact with my stepsister, it would be a constant reminder of the all the problems that I finally broke away from, and for my own sake and to be able to get over things that have happened in the past and move on with my life I had to make a clean break from it all. By not having to acknowledge any of the horrible things that happened I literally don't even think about them, just removed all trace of them from my life, I'm sure it's not healthy but I see it as a necessity to lead a happy balanced existence, you can't change the past, but you can sure as shit leave it behind you and forget about it.
I think you were collateral damage, like my stepsister was, they weren't asking you to choose between her and them, they just, for whatever reason, needed to move on from the past and unfortunately they had to leave you behind to do that, trust me when I say that decision was probably not taken lightly.
It sounds like you could do with a good talk with them to get all this out in the open, but if you go into it purely to tell them off about how all this has effected you, then you may as well not bother, it won't help resolve anything, because they sound like victims of this as just as much as you are. If you want to come to some sort of understanding, you need to be able to talk about what happened and be able to see each others point of view without judgement or derision and with no expectation of it changing anything going forward.