i think its fine to be with a 15 year old girl..as long as nothing happens sexually between you two..for legal reasons. heaps of my guy friends are around your age and a few of them have dated girls from highschool..one of which is still going strong (he's like 24 and has been going out with this girl for 5 years who is 19 now, but was 14 at the time). its just an issue of maturity to me. if you like her and she likes you..go for it my friend. Besides..she's only 4 years younger than you..my parents are like 6 years apart if that counts for anything. love has no age limit..sex however is a different story.
It's great that you're already getting along with the family. Goodluck to you!
"We all take different paths in life, but no matter where we go, we take a little bit of each other everywhere."