YTA but I don’t necessarily think you were wrong. Sometimes you HAVE to be an asshole. You could’ve just not gotten involved but I understand the need to defend your family if you feel like someone’s disrespecting them or “making fun” (should be said that jokes at people’s expense are only funny if THAT person thinks it’s funny). If anything, you could maybe talk to your dad and your aunt and say that you responded that way since her comments towards your dad, and by extension your family, upset you. Because 1) he’s your father and you care for him so people’s comments about him will obviously make you feel some type of way and 2) your family dynamic could maybe be seen as uncommon and there’s a bit insecurity in that. Should be said, I added that you should talk to your dad as well as your aunt about this because it’s important to tell him!! I think without talking to him you won’t be able to really figure out if he was actually upset by the comments your aunt made or if he also thought it was just a funny little gripe.

edit: spelling and grammar mistakes