I want to ask my cousin if she has been going to the gym every time I see her
Why ask that of her?
I don't believe her when she says she works out.
You should mind your own business and not insert yourself into peoples business. She might get offended and spread rumors about you to the family.
She's not going to talk shit about me, I'm her favorite cousin.
You don't know that. Better to keep your business to yourself.