This story is the sequel to Suzuka, where the titular character is a track star who finds love with the main character. Fuuka is their daughter. All I cared about is whether the main character would also have older busty women bothering him like the loud drunk college student neighbors in Suzuka. Well, this time around, those busty women are his sisters living under the same roof with the same aversion to clothing as the original. Good times, good times.
Hibiki doesn't like wearing clothes when it gets too hot. I suppose she doesn't consider Yuu enough of a threat to bother covering up at all.