First off I wanted to say that I'm so sorry about your situation. It must be awful. I'm sure you've been told anyway, but I'll tell you too - none of it is your fault.
There are things I can't remember. I cannot remember what my father looked like. I had cut contact with him when I was in my late teens, and within a couple of days, he had just disappeared from my head. It's the strangest thing, though I assume it's my brain trying to protect me. I know I'm not misremembering his behaviour. It's more complicated with my mother.
For a long time, I thought my relationship with her was a good one. Multiple phone calls a day, every day. I was encouraged to never have secrets, never keep things private, including things that were really none of her business and shouldn't have been shared. In return, she used me as emotional support, which she'd done throughout my childhood. When my SD had an affair when I was at uni, I was the one she phoned multiple times a day to cry down the phone about it. Now while I agree that this is a difficult marital situation, your 21yo daughter is not the person to support you through it, or to spill all the sordid details to. I pushed away the ick and the uncomfortable feelings. I genuinely thought she loved me and I told myself I loved her in return. But the more time I spent away from her, the more I realised that the whole relationship was arse backwards, that I didn't like her company, that I dreaded the phone ringing, I dreaded the obligation of making the call, that she always gave terrible and often damaging advice, and that she was quite often really unpleasant to me. When I had my own children and began to view her parenting through that lens the relationship fell apart completely.
I think it's really telling that your sister left home at 15. People don't do that if things are good. It's also telling that your relationship with your parents started to fall apart when you began to want more independence. For healthy parents, when your kids start to pull away, it's a sign that you've done a good job and it's time to move on to the next stage of your lives. Wanting to keep adult children on a leash is a bad sign.
Sometimes people who aren't cut out to be parents have children. I'm not surprised you're confused. I am still struggling to understand things and it's been a long time.