Yes, aunt by marriage.
To be honest, I can't say that I would consider doing what the op is talking about, but I don't think it's creepy, especially if the op didn't really know the aunt very well, and she isn't a blood relative.
The Aunt and Uncle have 2 teenage children together, the oldest being 15, so the OP has been this woman's nephew for at least 15 years. Blood or no, that is just creepy in my book.
And it is not just the OP I think is creepy by doing this. My brother dated a cousin by adoption once. We had been raised as cousins since babies so even though there was no blood relation involved it was just wrong (IMO) and the entire family (who is liberal by the way) was grossed out by it. The relationship didn't last but my brother still has to live with the backlash from it.