Chikan to nozoki: Neesan no shinshitsu (1991)

Yumiko, a classmate of Tadashi who just married his brother, lives under the same roof without knowing Tadashi's feelings, and they have a fierce nighttime affair. Of course, Tadashi is aroused by the two of them, and he doesn't understand either. One day, her husband, her brother, finds a letter addressed to Yumiko from a woman who adores her husband...