I think my little sister may be lesbian...?

There are signs that she may be, sure, but the absolute only way you can know is if you ask her. It may be awkward and difficult, but if you want to know, want to support her, and want to be a help to her, you should just ask. She may not even see that shes gay is that's what all this is about, so you could be a great help to her finding out who she is :)