こいしが全部脱いだので。
妹が裸なのに姉が服着てるのはおかしい(暴論)

前回→https://www.pixiv.net/artworks/83836504

Since Koishi is already bare.
The older sister still clothed while the younger sister is nude is not right (irrational argument)

Last time→https://www.pixiv.net/artworks/83836504