I'm new to wakeboarding and have a similar prob when the water gets rough. I don't bent my knees enough and allow them to be natural shock absorbers. maybe she feels a little intimidated when it's more 1 on 1 so to speak. maybe she wants to impress her brother, hard to say exactly what exactly a young girl/woman is thinking. the 'social" scene can be good at the same time can distract her from learning. I don't know your methods of teaching, and when it comes to family, sometimes some of us can feel pushed or like we're being told what to do, instead of seeing it as friendly advice. when it's more 1 on 1, maybe let her get the feel of it, get frustrated w/herself, maybe ask if she's ready for you to offer a few tid bits at that time, but also allow her to ask questions. in her own time, if she really wants to wakeboard, she will also ask your advice and opinion. From a girls stand point, she may not want to be "told" what to do. Suggestions are good though. Us females can be a complicated thing