It's Not Babysitting by anxioussquirrel
Kurt, Cooper and Sebastian are a close-knit circle of friends living their lives in fabulous New York City. When Cooper's teenaged brother, Blaine, comes to visit for the summer due to issues at home, all hell breaks loose...at least for Kurt. He's immediately attracted to the adorable guy, but refuses to pursue anything because he's Cooper's little brother and he's ten years younger. Will Kurt and Blaine be able to put their fears and insecurities aside for potential happiness?
This has to be one of the best fanfic stories I've read, period. All of the characters are well-developed and seem to jump off the page with their varying personalities. Even though the main focus of the story is Kurt's relationship with Blaine, there are other subplots that are just as interesting, but don't take away from what everyone was to see: Kurt and Blaine happy and in love.
I also appreciated that Kurt and Blaine's relationship isn't perfect. They both have their issues and must work on them in order to be good partners for each other. I definitely recommend this lovely story to any and everyone! :)