Lover-in-Law (2008)

Tsutomu, a young man living with his brother and brother's wife, spends most of his days helping out at the family flower shop. Unfortunately, with the head of the household away on business trips more often than not, the nights are pretty lonely for Tsutomu's sister-in-law, Mai. As she is coping with that fact in her empty bed one evening, Tsutomu ends up stumbling across the scene and helps her fulfill her womanly desires. But when Mai acts like nothing happened the next day, the frustrated Tsutomu takes matters into his own hands...

Animation