Mogitate girl: akiko (1985)

A woman who knows she shouldn't break off her relationship with her sister's husband. One day, her sister asks if she can find out if her husband has been cheating lately. Feeling that it would be dangerous to continue the relationship, the woman proposes breaking up with her brother-in-law. The brother-in-law, who still has feelings for her, reluctantly accepts. However, her body, which had been developed by her brother-in-law for a long time, could no longer live without a man. The woman seduces a colleague at work, a man she knows has a lover, and they forcibly have a relationship. In the end, the woman convinces herself that she likes to steal other people's things. However, the older sister begins to feel suspicious about the relationship between her husband and her sister, and the younger sister's new lover...