Saikin, Imouto no Yousu ga Chotto Okashiinda ga Temporada 1 Episódio 5

By coincidence, Mitsuki meets up with Yuuya, Yukina, and the others at a water park. But due to her worries about the chastity belt she covered with her pareo, she couldnt go in the water, and she was left sulking by the poolside as she watched the others happily having fun.