Recorder and Randsell Mi started, so I spoilered Re's post too.
AniDB wrote:The siblings of the Miyagawa family continue to stumble into misunderstandings regarding their appearance: The 180 centimetres tall Atsushi looks like an adult, but in fact is actually an elementary school boy. Getting confused with a teacher is one of the very few more charming problems he has to face. His undertall sister Atsumi, measuring only 137 cm, gets confused with a child prodigy, even though she actually is a normal high school student.
Original post under spoiler:
AniDB wrote:The story revolves around Miyagawa Atsushi and his sister Miyagawa Atsumi. 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.