Was i wrong for this?

me and my gf have been together for almost a year she has 2 kids. The biological dad sees them when he feels like too, so I'm like the only true father figure in their lifes. this morning the 2 year old daughter throw something at her mom and all the mom did was take her daughter's binkie away and threw it. I told the little girl that she was wrong to throw something at her mom, but then the she slapped me in the face and I told her that was bad and dont to it again. I got yelled at for speaking my mind, my gf said i'm not there dad and i have no say, But I love those kids like my own and I think they should have a father figure in their lifes.