If she is as dangerous as she sounds, it's better to do your best to remove yourself and others from the situation (if that requires restraining her or using some force, so be it) and to call the police-- let them deal with her, it's safer for everyone involved. Knocking her unconscious as a means to prevent further attacks is risky and can lead you to more trouble.
Anyway, that is unfortunate to hear, your sister seems like she needs serious help. It's sad to see people lashing out so violently, usually they hurt others because they're hurting in someway themselves and don't have the emotional maturity to express themselves so they resort this sort of behavior.
Yes, I would. I've already said as much previously in this thread. Still, a skinny 18yr old male can do more damage than the skinny woman in the video so I'd take that into account if we're doing a direct comparison.
And if it were a a large, strong woman attacking a very small/frail man, I'd feel the same. Or if it were a woman of any size trying to assault a man with a weapon/object of some sort thereby putting a man's life in immediate danger, I'd feel that an uppercut (and maybe more) was warranted.
Like I said, it's not specifically about gender but it is a factor in this incident because it involves a physically imposing man going all out on a small women that he could have very easily restrained in order to protect himself. On average, men are physically stronger than women (who generally have about 40-60% of a man's upper body strength and 70% of a man's lower body strength), so it's pertinent to the discussion. It's something we have to consider.
Going by the story, his life wasn't in danger, she grabbed his neck for a second to hold him in place so she could quickly spit on him, then she let go and stepped aside. I'm not trying to excuse her behavior but the uppercut was more life threatening than whatever she did to him. His reaction was not impulsive, either, because at least that would have been more understandable if he reflexively just hit her right when she grabbed at him. Instead, he paused (entirely unharmed, btw), then calming sauntered on over to her.