Mizuki Sugawara was boasting about his sister for a reason. Shinto-Ryu may be based on footwork, but his sister was a freak. Katsuki Sugawara was petite and she was as tall as a midget… Of course, people nowadays would call girls like her Lolita, there were even many who have developed a Lolita Complex, such as perverts.
Katsuki Sugawara was born with super strength and her power was boundless. When they were young, Mizuki Sugawara was always beaten up badly by his sister. Of course, after they grew up he no longer felt inferior as his skills began to be cultivated by their family tradition and he managed to surpass Katsuki eventually. Moreover she was his younger sister, so he could no longer be bothered by her.