concerned sister or transphobic

Priscilla
Ok, this is an honest concern. I'm not trying to start a debate or what's right or natural or anything like that. I need advice on how to answer my little sisters questions without pushing her towards either side.
Here it goes, my little sister says she is trans. She said she feels like a boy and the very idea of bras and panties make her uncomfortable. The thing about that is, is that I think she's trying to be something she's not. I'm not against the LGBTQ community at all, but I think my little sister is trying to follow the crowd. She was completely ok with being identified as a girl until Caitlyn Jenner came out and suddenly one of her friends come out and now she's coming out. Whether her friends are trans or not I don't care, I just don't want her to spend all her teen years believing this idea that she should be a boy only to go into aldulthood confused on what is a boy or girl. BUT then on the other side, if she actually is feeling this way I don't what her to feel she's any less important or loved because of it. This may be ignorance, but this is coming from a straight fully secure female who never doubted her identity or sexuality. No one ever needed to tell me I was a female or attracted to men, for me it was just natural. But how do I support my little sister, who just turned 14, without pushing her toward either side. As I said before, I don't actually think she is sure herself, I think she's following the crowd.  We've had many conversations about what it means to be a boy or girl in her definition, and because she's not so sure of her answer ( she alsways says something different when we talk) it makes me believe that she's not actually sure of this either.But if she is I want to support her in a way that is healthy. I know I sound really transphobic, but I just don't want her to regret her teen years or not understand what identity is.