Tomodachi no Neechan ni Koishita Hanashi

Rio is everything that Haruto wants in a girl: gentle, nurturing, and ladylike. The only problem is that she's the older sister of his best buddy Nao, and he fears that the difference in age means he's barely on her radar. If Haruto wants Rio to see him as more than her little brother's playmate, then he has to prove himself as a man in her eyes, no matter what it takes.

(Source: Irodori Comics)