I like my bff's brother...

So I have been friends with my bff for almost 10 years. We're 15 rn. I've known her whole family for that long too and I go to her house almost every weekend and it's summer now so a lot! So ever since I was in 3rd to 4th grade I had a little crush on on her brother (I was super young but still) the past year it's really been there for me too! Her brother and I have so much in common too! (My friend and the rest of her family think it's weird how much we're alike) he knows it too. I know that she wouldn't have a problem if we were together but the only thing is that he's 18.. He acts young for his age though and he's going into his senior year of high school. He shows all the signs that he likes me back but the age gap just confuses me so much. It's only 3 years but it seems like a lot now but once were 20 it won't be anything at all. I'm sorry this is so long I see him tomorrow and I don't really know what to do..... I also don't want to tell my friend I know she wouldn't have a problem and I know that would just make it super weird.