I think it's disturbing when kids learn to manipulate the system young. Like discovering how many adults jump into a frenzy if you claim abuse. It doesn't mention how old the younger sister was. She could have been eight or fourteen. It also doesn't mention whether their mother encouraged her to call the police. If the police thought it was a case of domestic violence, wouldn't the courts have done more than slap the boy with a fine?
Age was my first thought. If she was eight and her 15yo brother
punched her, yeah honestly that would be a pretty big deal. Could have broken her nose, knocked teeth out, etc. They refer to her as a "little girl" but don't specify what that means to them. Then again as Peter said she could be 14 or even 15, bigger than him, and those kicks could have been dangerous (could be "assault with a deadly weapon (shod foot)").
Another devil's advocate question- how old would would the boy and his sister have to be before those who take issue with the case would think it was appropriate that charged be brought against him for punching his little sister?