My girlfriend's younger sister is in heat and secretly keeps having sex with me, the worst kind of person. (Ogura Nanami SSIS-348)

Which would you choose: a girlfriend with small breasts who's hard to seduce or a girlfriend with big breasts who's easy to seduce? This thought came to me not long after I started dating my girlfriend. My proud girlfriend is difficult to seduce. But my younger sister seems very easy to seduce, so in my extreme frustration, I touched her big breasts. There was no strong resistance. Later, I went all the way, and I even secretly had thrusting sex with my super sensitive younger sister, Nanami.