Number of kids
One, two, three...
Two is worst. One elder often kills the younger one. Example; Romulus & Remus, Abel & Cain. Yoritomo & Yoshitsune, Hamlet, and many more.
Three is not worse than two. Example; Brothers of Karamazov, Three daughters of King Lear, Three sons of Mohri Clan.
One has few problems. Also that contributes to population issue. One can inherit all from his parents.