[TOMT] A book that's told through emails

It was like. told through emails. This girl was the main character. And she and this boy were both musically inclined, and I believe they were dating long distance until it's implied they met up irl, the boy got 2nd in some competition, and both realized how 'annoying' the other was and broke up. I also believe the girl's sister eventually came out as a lesbian, and there was a camp counselor who, when told, reacted with 'If i would've known that, i wouldn't have let her be camp counselor for all those girls' or something like that.

I'd like to read it again if possible