From infancy on, a second child functions sociologically as a third party.

Evidence

The second child is not only a fourth member of a relation but, sociologically speaking, also a third, with the peculiar effects of the third member. For, as soon as infancy has passed, it is much more often the parents who form a functional unit within the family than it is the totality of the children. — Georg Simmel, The Sociology of Georg Simmel, 4058