Yesterday, DS15 and DD12 were fighting. I was at work so I didn't witness it but DS told me that she took his phone and was reading messaging between him and his girlfriend and he told her to give his phone back but DD refused. DD is denying that this happened and said that DS just went into her room and grabbed her wrist (there was a red mark on her wrist), DS admitted that he did grab her wrist but only because she wouldn't give him his phone and it was annoying him.
How would you deal with this?