Yome Gurui ~Gifu to no Himitsu no Kankei~

The protagonist is a university professor who lost his wife and lives alone. One day, his son and daughter-in-law, who live separately, propose to live together. The daughter-in-law was originally the protagonist's student and often visited his home, which led to their marriage. The joint life with the son and daughter-in-law begins, but within a month, the son is assigned to work overseas. With a six-month deadline, the son decides to go alone. This leaves the protagonist and heroine to live together. The heroine, who had always admired the protagonist, crosses a line due to an unexpected event. This begins their forbidden life together.