Miboujin Gangu ~Boku no Oba wa Nikuyoku Dorei~

The main character starts boarding at his aunt's house. Her name is Kotomi and she lost her husband two years ago. There is another girl boarding at her house, Kureha, his cousin. One day, he sees Kotomi masturbating at her room. She asks him to lie by her side for a while to soften her sadness and he accepts it. He is supposed to lie by her side at first, but they finally sleep together.... Kureha soon notices their relationship and says to him, "If you don't sleep with me now, I'll kill myself...." Kureha's hatred doesn't settle down and it goes against Kotomi....