Surprise, your aunt's really an uncle!

Your situation seems particularly interesting, because it sounds like you haven't had much of a relationship with your aunt, seeing as how she's been overseas most of your life (right?). That means that it would even less important for you to know.

I would ascribe your family 'keeping it from you' to waiting until you were old enough and mature enough to be able to think it through - the fact that you're trying to figure out how you should react instead of just reacting lends credence to this. I say let the natural emotions you feel run their course, so long as you come back to the fact that your aunt is still your aunt, you just know more about her now. She hasn't changed since you found out, you just know her better now. If you have enough of a relationship with her, you might consider talking with her about it. If not, you might consider talking with your dad about it.

Jam it back in, in the dark.