Confronting my abusers

Your sister sounds like a psychopath. Mine is. She enjoyed tormenting me. I had to walk away from her several times. I hope to never see her again.
Psychopath’s don’t feel your pain. They feel nothing but emptiness inside themselves. Reacting to them is pointless and will only cause you more grief. The best thing to do is walk. Say nothing – just leave. Keep interacting with them if you must but keep it shallow and don’t tell them anything personal or confide in them.
You’re not the problem. Your sister and her enablers are. Unfortunately our society generally rewards people like this.
You end up doubting yourself and spending time anguishing over “what did I do wrong?”. Try not to.
Be very careful around these types of people. Keep talking here if you feel the need to. It helps to get it out. Don’t blame yourself for someone else’s behaviour.