Yoru Made Matenai.

Haruka is a sweet, innocent high school girl who is very excited to meet her new step-brother, Tsukasa. Since her father works overseas, Haruka has been living alone, but now that her father has remarried, she'll have little grade schooler Tsukasa with her while both their parents are out of the country. When they finally meet, Tsukasa is even cuter than Haruka had hoped! He wants to get her help with his homework and sleep in her bed…and undress her and make love to her!?

Side story of Haruka and Tsukasa is in Kakedashita Cupid - 6) Of Course I Can't Wait Until Nightfall
(Mochiron Yorumade Matenai)
Side story of Haruka and Tsukasa as adults is included in Suki ni Natte mo Ii no? - Extra Chapter 2