NTA, and I honestly don’t understand why people think otherwise.
It definitely wasn’t wise to talk to Mia about this during the birthday party, you should’ve waited for a more private time so you could sit her down and have a proper discussion.
However, the way she reacted was NOT YOUR FAULT. You are not to blame for the way she confronted Aurora, but you should’ve known how she would react.
I don’t think it was a good idea to confront her at the party and I don’t think that the stepmum adopting her is the best idea either, but does that make you TA? I don’t think so. It just makes you…a little stupid.
(Based on comments i’ve read it might be a good idea to not go through with the adoption too, but idk much about the topic so it doesn’t seem like my place to comment)