Rubén's comment is correct; the order in which names are entered will have no effect on the placement o the name.
If you list a number of siblings with no birthdates, they will be listed alphabetically.
If some have birthdates and some don't, those without dates will appear first (in alphabetical order), followed by those with dates (in chronological order).
Of all those with birthdates, any multiple births will be sorted alphabetically, (the exception being twins born on different days - e.g. just before and just after midnight).