Will you believe this? It's counter-intuitive.
In the 1911 census for England and Wales there's information on how long a marriage has lasted, how many children there have been and how many are still alive. That's in addition to information on names, ages, relationships, etc.
Suppose you have a family where the couple has been married for 25 years, they had two child who are both still alive and one of them, a girl age 21, is living at home.
Is the other child more likely to be male or female?
What do you think?
If a couple have two children the possibilities, in birth order, are:
Assume that each possibility is equally likely even though boy births occur slightly more frequently than girl births.
In this case knowing one of them is a girl we can eliminate the Boy-Boy possibility. That leaves three options two of which pair a girl with a boy and one with a girl.
It's twice as likely the other sibling not at home with the parents is a boy rather than a girl.
If you knew the girl at home was the elder, or the younger, there would be an equal chance the absent child would be male of female.