I grew up in a family of female bullies (mom and older sister). It is the reason I married a bully. Fortunately I wised up and divorced her.
Years later I was staying at the beach (the last time) with my birth family. My older sister had a daughter. I had a son. Age difference about the same as my sister and I.
My niece started bullying my son and her younger sister. First it was minor. Then I was on the deck watching them on the beach. My niece took something away from my son one too many times. He punched her in the chest.
She broke down and went crying to my sister and BIL. They confronted me and said if my son didn't apologize and change, then my son couldn't play with his niece anymore. I said, "OK, that's fine."
There is a lot more to the story, but as I wrote before, it was my last time staying down on the beach with them, and my son got one more trip and then he was not allowed to fly to them until he was old enough to leave if wanted (basically nearly 18).
I got a lot of emotional extortion over that, but my son (which HUGE help from my second wife) has grown up into a fine young man who does not bully nor tolerate bullies. Mission accomplished.