nothing to see here

Illegal? No. Unprofessional and unethical? Maybe.

Can you be fired for it? You bet you can. It doesn't have to be illegal to be a firing offense.

Unless you have a legally binding contract that specifically guarantees you more hours than you are getting, or you have more proof of gender discrimination than you've posted here (it takes a lot more than simply the other employee and the manager being female to prove that), then no, the company has no liability. They, not you, decide what hours are worked by which employee, and they are not obligated under the law to schedule you.

You, on the other hand, have just set yourself up to be fired as nicely as I've ever seen anyone set themselves up to be fired.