Is it wrong to flirt with an underage girl?

Anonymous
I should add that I've known this girl, literally, all her life. And I've regarded her as almost like a sister. But just recently she's kind of been flirting with me from a little to quite a lot. And actually I find myself to be quite enjoying the attention, and given 'normal' circumstances I would have done something by now. She is TOTALLY lovely.

Here's the clincher though, she's 13 and I'm 18, and that makes me feel like a pedo. Even without that complication, my younger brother (who's her age) likes her. So, what do I do? Just be cool and don't do anything?

Any advice is appreciated :)!
Updates
+1 y
Just to add: I have no intentions at all of 'acting' on it - for some reason my question has been edited and it lifts the emphasis away from where my question was. I just. Wanted to know how to handle the situation. appreciate the advice :)
Is it wrong to flirt with an underage girl?
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