Tomodachi no Neechan ni Koishita Hanashi to Neechan no Tomodachi ga Uzai 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.

Meanwhile, Nao grows frustrated with Sena, a loudmouthed friend of Rio's who's too nosy for her own good. But what irritates him more than the constant pestering is how she just can't see him as more than a little kid. Can he help her get a clue?

(Source: Irodori Comics)