Saikin, Imouto no Yousu ga Chotto Okashiinda ga.  

最近、妹のようすがちょっとおかしいんだが。, Recently, My Sister Is Unusual., Lately, My Little Sister Has Been a Bit Strange

A high school girl named Kanzaki Mitsuki recently gained a new older brother when her mother remarried another divorced individual with a son. Shortly after, her stepfather was transferred overseas, and her mother followed him there, leaving Mitsuki alone with her new stepbrother, Yuuya. Feeling abandoned by her mother, she kept a distance from Yuuya and struggled to get along with him or maintain any conversations with him. One day, an angel/ghost named Hiyori visited Mitsuki's body and attempted to make sexual advances towards Yuuya. Hiyori was the lost spirit of a girl who had died but could not reach the gates of heaven and achieve eternal peace. To do so, she placed a belt on Mitsuki called the TST that filled up every time Mitsuki experienced sexual feelings from Yuuya, whether it be during dates, kisses, or even sex. For each fill-up, Hiyori gained a step closer to the bridge connecting her current situation and the gates of heaven. If she wanted to rid herself of the bothersome Hiyori, Mitsuki had to make bold and seductive moves towards her own brother, Yuuya, despite him being only a stepbrother.