The default order is by birth, so entering any birth date information will put the siblings in the correct order. If no birth date infomration is known, but you know the birth order, you can explicitely set it by clicking on a sibling, clicking the "Change/delete" button on the left, then "Change sibling order" or something like that.
I don't actually know what happens if you change sibling order and there are birth dates. Is it even an option then?