My mum acted similarly to OP’s sister for years after my grandmother died and my grandfather dated, then married, a woman my mum had known most of her life. She cut off my grandad for about 10 years. Us kids didn’t see or speak to him for that whole time. We didn’t go to family gatherings for nearly a decade. My sister and I missed a lot of special time with our family because of my mother’s selfish and childish behaviour.

They eventually made amends and now everyone pretends it didn’t happen. She was a dick, tbh, and there’s some resentment there.

There will be consequences, but OP’s sister will bear the brunt of them. OP can and should invite whoever she wants and if they don’t show for their own petty reasons, that’s their choice. For everyone else, time spent together at those events is precious and limited. Make the most of it, OP.