After watching the video I agree with you, the little boy was very brave in his attempt to protect his sister. I got the feeling that after the thug attacked the little boy, the rest of those who were watching realized that she had gone too far, even for them.
The guy who’d pulled the attacker off earlier thought she was going too far even before that. Good on him for at least making an effort. I’ve seen too many videos like this when full on adults don’t try to put a stop to it.