I'm going to get in a fight with my mom for not letting me to go the gym when they let my older sisters to go the the gym
Why are your sisters allowed to go but not you?
I do not know why, that's why I want to fight.
I see your point, and it's fine to point out unfair treatment to your parents. If you never did then they may never change their mind.
You see my mom just shuts me out.
Telling your mother about her being unfair is fine, but you should refrain from getting into a fight. That will only worsen the situation and may not help at all.