Ha. I finally understand what's happening.

When adults sink to the level of a child while fighting with a child, it is because they lack the intellectual superiority that most adults have over children.

So my sister's assualt on a 12 year old when she was 16, meant that her intellectual development was not above that of a 12 year old. And she reacted to the annoying behavior of a kid as if he was her equal.

All these decades I have been dealing with someone who functions at the level of a child and giving their words and actions the significance of adult behavior. I think this realization will be a big change in my life.

Thanks everyone for letting me work out my thoughts on this forum.