Yagami's Family Affairs(1990)

Yūji Yagami is a high school student with an unusual problem. Yūji has developed a crush ob his mother, Nomi, who is abnormally youthful in both her looks and mannerisms. Nomi however, is oblivious to Yūji's Oedipus complex and unknowingly tempts him with her overly-passionate displays of affections to both her husband and son. The situation is further complicated when Yūji receives a love letter from someone else on Valentine's Day.