I'm the same person who asked for advice...

On disciplining my stepdaughter and if stepparents should be allowed to ground their step children or take away privileges etc a week ago... anyways, new problem...she now is pretending to kick her little brother in the head and pulling her foot away at the last second. She thinks she's being funny saying just kidding. Her aim is not that good. One of these days he's going to turn his head and she is going to kick him for real and I will LOSE MY SHIT. Her dad says she's just playing around and says stop that, more like asking instead of telling and she's back to doing it later. What do I do here? I've told her on numerous occasions to stop and she pouts and her dad tells me she's just a kid and is playing around. Help?