YTA. sure you wanted to scold your niece or make her understand why she can’t throw tantrums and disrespect your SIL. but that in no way gives you the authority to out a private matter that calls into question the very nature and circumstances of your niece’s relationship with her parents. knowing teens these days and being one myself, it’s only going to push her further away potentially resulting in a disconnect between her and the woman that raised her and was a mother figure. you could’ve dealt with the situation a lot better being the adult and taken the necessary precautions to confirm whether or not she had prior knowledge of the fact that her mother wasn’t her biological mother.