Fair enough, but personally I'd feel a dick if I decided to go out with a teenage girl, one night stand would be acceptable but hanging out with an 18 year old in a serious relationship would eventually do my head in.
Sounds f***ing creepy to me. Youv'e said you have known the family for years which means you've known this girl since she was a little kid in pig tails. Yuk!
Just knock one out over her if you must but leave it at that.