MIDE-051 My cousin is pretty and well-behaved, but very promiscuous.

MIDE-051 After spending a few years studying at university in Tokyo, my younger sister took advantage of the New Year’s holiday to come home and visit the family. I was shocked to see that after such a long time apart, the little girl I knew had become a beautiful, charming young woman. But…
I couldn’t believe my eyes. My sister no longer had that innocent look she once had; instead, there was something very feminine, very sensual about her. This was evident in her clothing (skirts that were too high, revealing her underwear; very thin tops that showed off her large, firm breasts).
This made me very uncomfortable, and thoughts of incest constantly crossed my mind whenever I saw her. What should I do???