Have you reached out to a therapist yet?
To answer your question more directly: family gatherings with someone who hurt you is hard as hell, there’s no doubt about that. But you’re allowed to not go. You’re allowed to do your own thing. You’re allowed to say why, though I would really think through possibilities and consequences before doing so. Family dynamics in the best of times are the difficult. Add abuse, it’s damn near impossible. I would strongly encourage you to contact a therapist and have some joint sessions with your sister.
I understand your anger towards her, but I wonder if the anger would better serve being directed at the relative who began the whole thing. Will they be at these gatherings? What is the relationship with them? Also, I know you’re hurt, you’re pain is very valid, but I also wonder if you can find some compassion for the 7 year old your sister was.
Friday already asked this but I didn’t see you answer. You say you want justice, what does that look like for you?