Unmei de Musubareta Koibito......no Azatoi Imouto to Issen wo Koeteshimau, Ano Hi made
運命で結ばれた恋人……のあざとい妹と一線を越えてしまう、あの日まで
A long-time promise with a childhood friend, Mitsuha Mahiru, is fulfilled when they reunite and become a couple. As they begin living together, Mahiru's younger sister, Saya, joins them as a housemate. One night, Saya enters the protagonist's room and kisses him, asking them to keep it a secret. From that point, their shared living situation becomes complicated, marked by hidden interactions, until the protagonist ends up crossing a boundary with his girlfriend's sister.