Lana, IIRC you said that you have a relative who is trans so I'm going to word this very carefully:
Teenagers in general have a tough time trying to fit in or trying to find out who they are, for trans this is even more so. Going around and claiming that someone who says that they are trans isn't and is merely doing it for the sake of being "cool" is a really dick move. This is made even worse by the fact that they are more likely to suffer from depression ...Gee, turns out that when the society (or even your own family) doesn't accept you for what you are it can cause problems for your mental health?!