u/bbkn7

I initially thought that they'd obviously be shorter since they were younger. But then I realized the twins are 16 at the start of the series. Girls will have typically reached their maximum height at Kagami/Tsukasa's age.

Both Kagami and Tsukasa are just 1cm shorter than the national average height for Japanese women (158cm)

So it's their sisters and mother who are unusually tall.