I have been treated horribly in public by my older sister and no one stopped her but instead was intimidated by her.

It's interesting reading the advice of not saying anything. I wish someone had when I experienced it, at least made a note of it somewhere so I would be able to have written proof instead of people too afraid to say anything leaving me basically alone to deal with it which continued to occur to me that continued to be toxic.

In my experience when someone behaves badly like this people are too afraid to say anything or even choose to blame the victim instead of the person who is acting out so badly.