Imouto Hame ~Anna Yatsu ni Watasu Kurai Nara…!~

The story revolves around the protagonist's younger sister, Maki, who used to be affectionate and close to him. However, she has become distant and somewhat rebellious lately. The protagonist suspects that she may have a boyfriend and becomes anxious. Fueled by insecurity, he decides to secretly enter her room while she is in the bath to investigate. As he attempts to check her phone for clues, he is caught off guard when Maki confronts him. At his sister's questioning, a strong possessive feeling floods through the protagonist.

[Adapted from the official website]