Tomodachi no Nee-chan ni Koishita Hanashi

One day, Harusho Arisawa, a second-year junior-high boy, went over to the house of his best friend, Hisashi Misumi. That was when he meets and falls in love at first sight with Rio, Hisashi's beautiful college freshman sister. Is it just an innocent boyhood crush or...?
(Source: Comic POOL, translated)