Question

i would like to add an elder sibling to an existing child . How to do it

Answer 1

Go to the existing child. When no categories for entries/changes are open, you will see "Click to add relatives to [the existing child]", and an option below that is "Add brother/sister". Click on that, and then a new box will open for "Sibling of [the existing child]". Now you can input a new sibling. He/she will automatically have the same parents as the existing child.

You can change one of the elder sibling's parents, if he/she has a different mother or father; how to do that is explained elsewhere).

Answer 2

Set the birth date of the siblings, that will order them. If you don't know the birth date, use some fake date (to re-order the siblings) and then delete it.