Hey Have Hope.
I have only one sibling and he and I have a brittle relationship at best. He is basically a good guy, but he's the kind of guy who goes into any situation knowing what's best for everybody- particularly when it comes to family members. And he isn't interested in listening to what others might want or think about a situation.
It sounds like you have some similar strong willed dynamics in your own family. Your sister has a lot of stuff going on, but it came out as undependable for you. And she has a history of seeing to her own needs at the expense of others.
Your mom's insistence that you guys have a mediation strikes me as a difficult situation. Like forcing children to play nicely with each other even if they aren't ready. Maybe you need a little more time and space before you're ready for that? As an adult, you're being directed what to do, and who likes to be in that position?
No answers for you. Just sympathy. Family structures like that are difficult, especially where there are old expectations that have always been in place. Who else in your life would you allow to be like that with you? And yet family can be a little different. I have a polite relationship with my brother, but it's not close. It's hard to navigate these kinds of relationships where you need better boundaries, but don't want to risk them falling apart.
Hope you can find peace for yourself.