can i get in trouble

I am doing private duty for a 14 yo girl with leukodystrophy 65 lbs, gets weak fast, who can walk with unsteady gait and is a and o 3 but at a 9 yo level. So please someone tell me why did mom leave her 3 yo active stepson with the 14yo to watch while she went to church. I was angry and told the girl that's he can't watch him, she just got fresh and stated you don't have to watch him I'm watching him(nj state legal age is 13) I esp. Don't like the way mom did it, she just went out the door without telling me. Mom does not speak english but girl speaks both, she could have told her to tell me. Anyway, if something happened to the 3 yo would I have been responsible since I am the adult in the home at that time,, the girl, or mom