Which would you choose: a "small-breasted girlfriend who can withstand pressure" or a "busty sister of a girlfriend who can't withstand pressure"? I started thinking that way soon after I started dating my girlfriend. I'm proud of my girlfriend, but she's very guarded when it comes to sex (cry). On the other hand, her sister seems quiet and timid, and I was so aroused that I ended up touching her big breasts... What! She didn't resist strongly (lol). From then on, I decided to have sex with my super sensitive sister Nanami in secret!