Natsu no Arashi! - Season 2 Episode 11 High School Lullaby

One morning when Jun woke up, her body was that of a grown woman. Arashi was the only one that noticed right away, though none of the others knew who she was. To get out of an awkward moment, she tells the others she is the older sister of Jun, by the name of Ryoko. When approaching Kaya to tell her the truth, Jun learns that they are planning a surprise birthday party for her. Jun ends up helping the preparations for the party. While helping the preparations, Hajime compliments on how beautiful she is, but Jun gets annoyed as the older Jun is being compared to the younger Jun.