My little sister has recently grown into a mature body. When I saw her casual panty shots and her unguarded chest, I couldn't help but get an erection! Then she asked me, "Big brother, are you excited by me?" and I couldn't help but reply, "You're excited by me too!" It's true that I, a virgin, got an erection. But it seems that my little sister was excited by my erection... Our ugly argument ended up crossing a line that should never be crossed... We both think that it was just a one-day incest.