Is it wrong?

I'm a 21 year old guy, currently dating an older woman. It seems to be going pretty well. I like her quite a bit so I'm happy at the moment.

Every now and then, this girl I don't know very well pops up on my Facebook, just for a random chat. Never lasts long; it's just the generic 'hey how are you?' sort of thing. This girl is 14.. and is the sister of a girl who was in my year in school.
Now I want to be clear, I would never touch her. Besides the fact I hardly know her, the age gap is way too big and her sister would kill me. I just can't help noticing that this girl is extremely attractive, however.

I'm loyal to the girl I'm seeing so there is never any issue there, I just feel horrid and dirty for finding a girl 7 years younger than me attractive. I've never had indecent thoughts about her, I just recognise that she's really quite beautiful. Is this wrong or am I over thinking a bit?