AITAH for purposefully embarrassing my sister at her birthday dinner?

I (31M) am really close with my mom (56F) and I always have been. I tell her everything. My older sister (35F) can’t say the same. For as long as I can remember, they’ve had a strained relationship. I try my best to stay out of it while maintaining both relationships.

I lived abroad for the last three years so I wasn’t around for my sister’s birthday celebrations during that time. I’m back in the states now, so last Saturday I was finally able to attend a birthday dinner of hers. This was going to be the first time I would be meeting most of her friends.

I immediately knew I was going to have a bad time when I greeted with something like “so we finally meet the second one who survived the wicked witch, huh?” coming from a stranger speaking about my mother.

I asked him what he meant, and he said he had heard plenty of infamous stories from my sister. I’m still not exactly sure what stories he’s talking about, but I decided to tell one of my own. My husband is chronically ill. He moved in with my mom and I for about a year before we were even dating, just best friends, so my mom could help out while he recovered from a major surgery. That’s just the type of person she is: warm, kind, generous.

I guess I could’ve left it there but I said “That’s who my mom is. I don’t know what kind of bullshit she’s been telling you, but you should probably be friends with someone who isn’t a liar.” My husband and I left immediately.

My sister is now saying I’m an asshole for purposefully embarrassing her at her own birthday party, and that we’re allowed to have two different perceptions of our mom and she’s allowed to vent to her friends. I’m too angry to see things very clearly right now. AITAH?