I'm so sorry to learn about your sister melisssad81. I can imagine it being an enormous trigger to have to talk about that in therapy (or at all).

If it were me I would tell your T beforehand your concerns about why you don't want to get into it (just exactly like you did here), so that he knows where and when to steer you. You deserve to feel better in your life (even if only marginally) and like you said, the only way to do that is to get it out. Easier said than done, I know.

Lots of hugs for you.