I asked to stay with my brother's house and the ending was unexpected

To take the university entrance exam in Tokyo, I was taken care of by my brother and his wife who lived in the city for a while. My sister-in-law, Ai, had to monitor my studies because she graduated from a famous university. However, I looked at Ai's sophisticated sex appeal and masturbated in shiny luxury underwear, and I, a country boy and still a virgin, could not learn. Then, Ai-san came up to me and said, “Didn't you see it yesterday?”