Hiroki invites his senior and girlfriend, Yuuna, over to his house for the first time. The atmosphere is good when they're alone, and just when they're about to have sex for the first time, his younger sister Nanaka bursts into the room. Frustrated that she lost her brother, Nanaka challenges Yuuna to a breast contest. While acknowledging the appeal of both, Hiroki chooses Yuuna, and Nanaka admitted defeat on the spot,, but the next day she crawls into Hiroki's futon...!!

[From Getchu]