President Yukihira! Please Teach Me Naughty Things♡/official summary:
Chiharu Yukihira and Sara Hasumi are like family. When Sara's parents died, Chiharu was the one who took her in and helped her achieve anything she wanted. To the young Sara, Chiharu was like a big brother but now all grown up, Sara wanted to be Chiharu's wife. To do that, Sara wanted to be more mature. And it seems like Chiharu is all willing to teach her about it.