My wife has one sibling: her sister. She has no brother.
I pointed out to her that her father was the very first in her family line to not have a son. Her father's father had a son. His father's father had a son. And so on: if you went from her back in time to the origins of man, there would not be one single man in that line who had not sired a son. Until her father broke the streak.
She hasn't talked to me in hours. I may have inadvertently blown her cerebral cortex.