あねいも～アイとHのステップアップ～

The main character is Tohno Kazuya, a senior in high school who lives at home with two girls his family took in ten years ago. One is slightly older than he, named Manami, while the other is slightly younger than he, named Madoka. The three, for all intents and purposes, live by themselves in a rather spacious house in the suburbs, since Kazuya's father works abroad and his mother is deceased. The setting is in the middle of summer break, beginning in June and ending in August, and the premise of our story is the budding...attraction between Kazuya and either Manami or Madoka.

[From moroboshiyuumei review]