Three years have passed since we got married, and my wife got pregnant. She said she wanted to give birth at her parents' house, so she decided to go back to her hometown. I thought it would be nice to live alone for the first time in a while, but my wife's sister came to help with the housework. When I met my sister-in-law for the first time in a while, she had grown up, had a great figure, and was very attractive. Although I knew it was wrong, I couldn't help but see her as a woman. I couldn't relax when I thought about the two of us being alone in the same house. Then, I saw my sister-in-law masturbating, and I couldn't help but approach her...