hey deadinside. not not many people here actually answered the question huh?
your sister isn't trans. that teacher should be fired. and YES this younger generation of woke trans movement is asserting gender roles while simultaneously telling cismen they are toxic for being masculine and ciswomen suffer internal sexism for being feminine. because narcissists are big on cognizant dissonance.
sex hormones during prenatal brain development determine whether you're a mentally a man or woman, non-binary, cis or (in less than 1% of the population) "trans". during formation, the SRY gene either activates and the vagina becomes a penis, ovaries turn to testicles and drop down, encased by the labia that become the sac, or it doesn't and the vagina and all that remains. and sex hormones influence, mostly, how that goes.
but the brain isn't developed yet. it develops much later, after the genitals. and sex hormones also influence whether it develops masculine or feminine. most of the time, it all follows a system that our DNA developed over millions of years. but rarely, there's some lack on communication somewhere, and the brains of a female fetus, for example, may be more masculinized. you got yourself a possible trans boy. but that won't make itself certain until puberty, when another surge of sex hormones influence and mold the brain AGAIN. which is why many feminine girls suddenly have a tomboy phase through their teenage years. especially since androgens tend to be a bit more "louder" in their influence than estrogen.
if the teen only becomes more masculine all the way into adulthood. Trans guy. If the teen enters adulthood, looking back and going "what the heck was I thinking lol?" that's a girl.
so gender is in the brain - along with everything else about your personality. and sex is determined by your SRY gene. but you are right about everything else. I'm telling you. get that teacher fired.