Imouto Rule

Momono and her husband have moved in with her brother-in-law, an illustrator of adult art.

After being caught masturbating while looking at one of his drawings, she is forced to agree to a specific condition in exchange for keeping the incident a secret: she must live as her brother-in-law's younger sister. As she endures the humiliation of this role, her body begins to respond in unexpected ways...

(Source: Fanza, translated)

Note: Published in comic Anthurium Vol. 41.