Hi, Emily!
I came out to my brother and sister a couple of months after I went full time, and they were perfectly cool with it. Of course, I'm not nearly as young as you, so it wasn't nearly the worry for me that it was for you. Still, it's nice to have the support of your siblings.
I also confessed to my sister that way back when, I'd "borrowed" some of her clothing from time to time, and she just laughed it off. She also told me--and I found this hard to believe--that she'd never noticed any funny business going on. That was lucky for me, because if she'd started asking questions, I'd have been in big trouble with our parents.