Bought to attention

My boyfriend (39m) gets mad at me (37f) when I bring attention to something SD (15) did wrong. What happens is he confronts her, she denies, gets upset, may or may not bring her mom into it, then he gets mad at me for saying anything.

Am I in the wrong for bringing to his attention?

I NACHO as much as possible. But I am nosey, which is what gets me in trouble. & I have a very hard time letting her get away with things.

SD lives with us full time. SO has always been the provider, and relied on BM to parent. BM is not a parent but rather a friend. SD plays parents against each other, taught to her by BM.

SD and I got along wonderful, we were best friends, the first two years, until BM got jealous. The last two years have been a downward slide.