Nee-chan's Secret

While there isn't a explicit tag this chapter, Umi-neechan defiantly gives off the older cousin or osananajimi vibe.
Page 15, vanilla, mating press, leg lock, heart pupils.... Umi-neechan sure is one horny Onee-san :)

Yuu-kun is always frustrated that the older Umi-neechan has always treated him like a little kid since they were younger and years later he finally see's the opportunity to have some payback and tease Uni for once after he finds the "extremely well hidden and totally not obvious" SM Maniac adult magazine under Umi's bed. And of course in classic Onee-san fashion, Yuu's plan doesn't go how he wants and Umi is still the Onee-chan and Yuu continues to get flustered by Umi in the way only an Onee-san like her can. Good stuff!!

-@MHM-Dono, from Umi's mom to comment how much Yuu has grown and the fact Yuu visited and stayed over at Umi's house plus the picture give me more relatives vibe but there's also the possibility they've been friends for so long because their two families are super close and friendly.
Osananajimi or older relative, no matter what this a massive win for all us Onee-san connoisseurs!

"He Saw ... He Came ..... BUT He DID NOT Conquer!"

Quite Possibly, Umi-nee was actually waiting for some time for Yuu-kun to become more assertive, or even more manly, even to the reveal that she secretly fantasized that he'd "rape" her (perhaps not the full-on degradation type of rape, but at least he'd force himself on her). Only flaw in that little scheme turned out to be that, try as he might to assert himself, Yuu-kun is (figuratively speaking) "ten years too early" to dominate or conquer such a Sexy Onee-san like Umi-nee.

And as a song from the 1970s that is still still popular even today said it: "That The Way (Uh-Huh, Uh-Huh) I Like It (Uh-Huh, Uh-Huh)!"

EDIT: Now that one looks more carefully, those magazines Yuu-kun found on Page 2, plus Umi's "reaction" to his finding them strongly suggests she used "Reverse Entrapment" strategy (or something) on him. And It Was "SUPER-EFFECTIVE!"

Here's Another one "APPROVED!" and lodged in the "Sexy Onee-san" Personal Collection.

That is one naughty sexy onee-san!