Click on one of the siblings, you'll see a button 'Change or delete' (left).
Now click 'Change sibling order'. The rest should be self-explanatory.
NB: as soon as the siblings have burth dates, this option will not be available, because the order is given!
Copied from the FAQ (!!): Add the birth date (or even just the birth year) of each brother or sister, and they will be reordered according to age. If you don't know the birth years for some people, click 'Change or delete' then 'Change birth order' and click to move them as appropriate.