My Little Brothers Best Friend

Kyle and James are two brothers separated by a year long age gap. They are almost polar opposites
Riley is Kyle's best friend. They have been since they were before they were even born
Both families are stupidly close practically one family merged into one

What happens when James comes home after being away all summer to see Riley has grown up a lot in the 2 months he was gone
What happens when Riley learns the truth behind why no guy has ever attempted to take her out or anything
And finally what happens when Kyle notices something off between the pair