Should one stop taking one's sister to a club that has a male coach after there are signs of her reaching puberty?

Obeying one's mother is obligatory in what is permissible, as long as she does not command a sin; for there is no obedience to a creature in disobedience to the Creator. The pure Sharia has prohibited aiding anyone in disobeying Allah. Your sister is forbidden from mixing with a non-mahram man, and you must advise her and explain the impermissibility of her adorning herself in front of a non-mahram man who trains her. If she does not desist, then you must stop driving her to this club so that you do not aid in wrongdoing.