If there is no birthdate for siblings, then they will be arranged in the order in which they were added to the tree. If you have a mixture of siblings with and without birthdays, the ones with birthdays will be first (in order of age), and the rest will come after in the order that they were added to the tree. Finally, as you know, if everyone has a birthday, they will all appear in order of age from oldest to youngest from left to right.
I do not think there is a way for you to change this behaviour.
