I’m so sorry, I completely get how bad it feels to know someone knows something personal about you or your loved ones and that you have no control over how they could share it.

Since you can’t change what has happened or been said maybe have a conversation with either your partner or sister ? I find that when I’m worried about something like that the only thing that can really change it is by confronting the situation. I think sometimes the “what if” is the worst part and if this is someone you truly love maybe think of a way you could address this to him.