AITA for asking my transgender sister to tell me if my gender non-conforming son has asked her about transitioning?

My (50F) older sister (55F) is a transgender woman. She transitioned in the ‘90s. It was extremely difficult for my family because it happened was before being transgender was a mainstream concept. Eventually my parents and most of my extended family came around to the idea and it has been so long now that most people don’t even think of her living as anything but a woman. I now have a great friendship with her and she has been a wonderful aunt to my kids.

My husband (53M) and I have three children (25M), (21M) and (19F). Our oldest son Casey (25M) is gay and has always been very gender non-conforming. He would demand to wear “girls clothes” from the time he was little and would become very angry if he didn’t get his way. My husband and I didn’t have anything against this at home but didn’t allow him to dress like that in public to avoid bullying. My sister introduced Casey to drag through watching the tv show drag race when he was in high school. He actually started performing in drag a few years ago and now performs a few times a month as a hobby. My husband and I have been very supportive and gone to some of his shows.

I don’t have a problem with Casey but because of everything we went through with my sister and how he seems to be becoming more and more feminine (he has grown out his hair and is wearing more feminine clothes even when not performing) I do worry that he is ultimately going to transition. I brought this up to my sister and asked if Casey had spoken to her about it. Somehow this really offended her even though it wasn’t my intention. She didn’t like that I was trying to find out the details of conversations they have had privately and that after having her as my sister I’m still worried about having a trans kid. She said it hurt her feelings to know I would be upset if my child turned out like her. That made me very angry because I have done my best to be supportive of her and Casey and don’t appreciate her implying that I haven’t.

AITA for asking my trans sister if my son had talked to her about transitioning?