It sucks that a fight between two people can also create rocky relationships with others. :/ I have been in this situation as well, it's never fun when people feel like they have to "take sides".

Your mother sounds like she is quite a toxic individual and really wants to keep her grasp on your step-dad and sisters. If I'm understanding correctly, this seems like a lose-lose situation. You could either end your relationship with your toxic mother, but also potentially lose your step-dad and sisters in the process. Or make amends and still have to be in a relationship with the toxic mother. I really feel for you.

And just to clarify, I don't see you having issues with people on the site here. Just with family stuff, which I am sure most of us can relate to at some point. I hope you are able to come to a decision that brings you the most peace.