They were all the sons of Kronos, so they were brothers. The kids, with their sisters, went in this order: Hestia, Demeter, Hera, Hades, Posiedon, Zeus. But, all, except Zeus, was swallowed by Kronos as soon as they were born. When Zeus got his dad to throw them up, the considered themselves how old they were in the order that they were belched out by Kronos. So they then considered them this order: Zeus, Posiedon, Hades, Hera, Demeter, and the Hestia.