It sounds to me if she "knew you always wanted to be a boy" whether she's ignorant about it or not, she had some understanding. I bet she'd be pretty easy to talk to, and help understand what being Trans is all about.
I have a rather big family on my dads side, 6 aunts/6uncles. It didn't really "stun" anyone in the family, being I was a huge tomboy from the time I could walk/talk. Majority of my family is 'accepting' as in, they respect me and love me, whether they understand or not, I do have one aunt, who, from day one (about 14 years old) was the first to call me her nephew. It came pretty natural to her, and made me WANT to open up to her. We've always been in some what contact, but I got the solid chance last summer over a family reunion to really thank her for always being so accepting and loving. I don't think she ever realized how much it meant to me as a kid, to have her introduce me to her friends as her nephew. And the fact back then, she wasn't told much, other than I was trans by my mother.
Sometimes you'll get those blessings in disguises, of people that just "get it".
My advice - go for it, try to have some one on one time with her, and help her understand.