Aside from the other woman aspect, is the notion that this is your uncle, albeit, technically not blood related, just by marriage.

You are 27. There aren't other men around, who are more available, emotionally and legally? (legally, meaning, not married) Your aunt could, legally turn around and file a lawsuit against you, depending on the state that you live in. Which is a big risk for those who plan to be the other woman in a married man's life. It's called alienation of affection lawsuits.