OP I’m sorry this is causing you such anguish. Is your sister a lesbian? A lot of young lesbians are under pressure from an unaccepting society and a hostile XGBT community. God knows I struggled enough 20 years ago when lesbians were just dirty and disgusting, but now we’re painted as positively evil, mainly for not wanting sex with males who identify as women.
OP I don’t know if it would help, and of course your sister might be straight, but if she’s not, or you suspect she isn’t, could you talk to her and gently tell her that you love her as who she is and you’ll support her whatever, but you think she should wait a few years before she acts.
I think I would have wanted to transition if things were as they are now back then. I was scared of being different, of not fitting in, and terrified that I might be a lesbian. I am, and I’m glad I am, but it was hard to come to terms with then. It’s amazing how much you change when you’re a young adult, I don’t think I really fully matured until I was in my mid/late 20s, and decisions you make at 18 can look very very bad 7 or 8 years down the track.
Anyway OP, I’m rushing here so sorry if this doesn’t make much sense, but good luck. It’s completely understandable that you’re worried, anybody would be, it’s just common sense. Can I suggest you look at Transgender Trend? You might find some helpful information and some good sound advice.