You can change the order of siblings by clicking on one sibling and then the "More Options" button on the bottom of the sidebar. Now, you can choose the "Change sibling order" action and make the sibling move left or right in birth order.
However, this is only possible when none of the siblings you want to order has a birth date added. For example, you have "Lara" and "Mary" that are "Born before 1970" and you have added "Lara" first, but know that Mary is the older sister. Despite both sisters having "Before 1970" as their birth date (which is not a date but rather a time span), you can't make Mary displayed as older sister on the tree.
Hope this helps!
