(((gentle hugs)))

This sounds like a very complex relationship issue. It is your sister who is engaged and planning a wedding. The fact that she remains so close to your ex's family... could it perhaps be that there are unresolved issues that you haven't yet shared with your sister? Does she have a true understanding about what happened during your manic episode 4 years ago - and did the episode impact her in a way that you are not aware of?

My questions are not meant to judge - only to try to better understand your situation.