AITA for telling my niece she does not have a степfather.

I (52 M) am married to my wife of 31 years. We have a daughter who is about to turn 16 and a 15 year old son. My wife has a sister and a brother. Neither of them are married. She now has 4 kids. My brother in law is a widower. <br><br>My wife's sister is my niece's mother. She is a single mother. Never been married, has a different man in her life every other week. Which I don't have a problem with. I do have a problem with the fact that she parades them in front of the kids. My niece is 9 years old. She is usually quiet. She talks but I don't think there is anything wrong with her but she has never been the talkative type. She is a very sweet girl though. <br><br>She was over a couple nights ago along with my wife's brother (29 year old autistic son) and his daughter. We were having dinner and the conversation went to her mom's new bf. The kids were talking about what they do and don't like about him. My niece out of nowhere said "My stepdad is so nice". Nobody said anything. But inside I was raging. Just the fact that she called him her stepdad pissed me off. <br><br>All in all, I have been the only father figure in her life. I never treated her like a niece. I treated her like I would a daughter. When she said that I was so mad. I was angry at my sister in law for doing this. <br><br>I told my niece "I am your uncle. That is not your stepdad". I don't know why I reacted so harshly. I honestly don't know how she took it. She did not say anything and we kept eating. But after dinner she was upset. She told her mom what I said and my sister in law was very upset. The rest of the family was there and they tried to get me to apologize. <br><br>She is a very sweet girl. I just hate she called him her stepdad. I don't want her thinking that I don't care. But I don't want to be treated like I am nothing either.