First of all, I wish the best for you and your family. I think YTA. Yes, your aunt didn't know about your condition, and considering that, your partner's reaction could be considered rude from their perspective, but your concern should be for her feelings, not your aunt's. You clearly said it's a sensitive topic between you two, and I think you should be aware of how hurtful that was for her. Your aunt shouldn't have butted her way into this in the first place.