Have you tried asking your dad to overturn the decision? >
But in all seriousness, the best you can do is explain to her that it's a youth organisation, that no one is required to join up upon entering and that contrarily to popular misconception, we're not trained to fight. As for Marksmanship, you can point out that there are several olympic disciplines that revolve around shooting.
A few arguments I used when convincing my parents:
-cadets have a lower crime and drug use rate compared to the "normal" youth of their age.
-Free and fun camps in which you are renumerated.
-possibility to receive a scholarship
-Great life experience and looks great on your resume.