"How many siblings do you have?"
A lot of languages make no sense. First they make up rules then modify them due to the problems created by the first set of rules. French is similar too because the masculine/feminine gender divide of le and la, un and une, breaks down in a lot of situations. A man can be called une person (a person), while he is un homme (a man). That means he changes his gender depending on whether he is addressed a person or a man. It makes absolutely no sense.