When she asked about a lock on the door, I expected it must mean Shii-chan liked to do the same kind of private dancing in her bedroom as Hoshihara does in the shower. But dancing with others is okay too.
@Lilliwyt
That just makes it better. Twins>siblings>second cousins>unrelated>first cousins.
First cousins are last because they're usually just poor man's siblings, where the mangaka doesn't have the guts to go all the way.
wait, wait, wait, doesn't she sleep on a bunk bed with her sister? you telling me the cousin sleepwalk out of the room, arrive in that room, not atk the sister, but climb up and atk her?