Recorder and Randsell

Recorder to Randoseru (リコーダーとランドセル, Recorder to Randoseru, lit. Recorder and Backpack) is a manga written and illustrated by Meme Higashiya. It was adapted into an anime in 2012.

Plot
The manga revolves around Atsushi Miyagawa and his sister Atsumi Miyagawa. Atsushi looks like an adult at 180 centimeters (about 71 inches) tall, but he is actually an elementary school boy. Atsumi looks like a little girl at 137 centimeters (54 inches) tall, but she is actually in high school.