IENE-372 Sister's friend under a kotatsu, panties visible, erection discovered, she plays with it

Aihara Tsukiha's sister brings a friend over when the heating breaks, and they end up sharing a kotatsu in the bedroom. The friend says it's too hot and dives under the blanket, where she sees the sister's panties and notices the sweat. She teases the erection and eventually messes with it.