I boxed before and after I was baptized. I don't remember any big fuss about it..... this was back in the late 1950's of course I wasn't competing just a couple of friends who liked to box in our basement.
I also played drums in small bands.....no problem back in the day.
Times have changed and this religion is now more then ever....... hard core.
Akid48 stay cool with your Aunt (your legal Guardian) but definitely avoid getting baptized. Just tell her may be after you finish college in the meantime see if you can get back in the school band. There are scholarships for Tuba players. Marching bands are a big deal at the college level.
Take your sport where you can find it.
Basket Ball works as a one person sport also other kids can hear you making your shots and will wander over to see what's going on.
Soccer works for one person as well. You get better with competition but you also need to first learn how to control the ball.
Lifting weights you can do in your room.
Remember you are not obligated to join your Aunts religion. And I don't think she can forbid you to engage in after school activities. A School Counselor may have the answer re your rights within the school structure.
Exercise your freedom to pursue healthy and creative goals.