Ok well i can't say I've ever been in your situation....but here goes

First off I can understand why you are so worried about comming off as a perv, but you guys are six years apart, I know plenty of people who date/marry people upto 10 years younger/older. I mean if you were say 30 and she was 24..that wouldn't be so bad would it?
But now think of a sixteen year old dating a 10 year old, or a 18 year old with a 12 year old is certainly... wrong.

What I'm saying is at this time dating her would be inappropriate. But I don't think you need to cut things off with her and your friend. If you really love her, then stay her friend, and give it some time. She is very young at 15, let her grow a bit, and then see where it goes. If what you feel for her is real love, and later she too feels the same, then you could date. But at 15 most people don't even know what love is.

Your behavior toward her has been respectful, you haven't made any advances, which is good, and pretty much takes you out of the perv category. You should know that she may never think of you anything but an older brother. But live your life, and please make up with your friend, cuz if your gonna feel shitty anyway, then you might as well feel shitty, and still have your friend, and keep in touch with his daughter. For as long as you feel you won't like start to behave inappropriately toward her, there is no need to runaway.

I say continue to be close to her, and act like you always have, she doesn't know
how you feel and niether does your friend, I don't think you should tell them either...cuz I mean that would make things WIERD!!

Good luck Tony...hope things work out no matter what you choose to do.

__________________
And since you know you cannot see yourself,
so well as by reflection, I, your glass,
will modestly discover to yourself,
that of yourself which you yet know not of.