After getting married and moving into my husband's home, I began living with his sister, Mitsuki, who had attended an all-girls school. However, I often noticed Mitsuki's gaze towards me, and then there was the unexpected morning kiss. I thought it was just the mischievous act of a seductive girl, but then I was told that before our marriage, when I was dating her brother, she had wished, "I hope such a sister-in-law comes to our house."