Briareus sibling Cottus
Principal sources
The Theogony
According to the Theogony of  Hesiod, Uranus (Sky) mated with Gaia (Earth) and produced eighteen children.  First came the twelve Titans, next the three one-eyed Cyclopes, and finally the three monstrous brothers Cottus, Briareus and Gyges. As the Theogony describes it:Then from Earth and Sky came forth three more sons, great and strong, unspeakable, Cottus and Briareus and Gyges, presumptuous children. A hundred arms sprang forth from their shoulders, unapproachable, and upon their massive limbs grew fifty heads out of each one’s shoulders; and the mighty strength in their great forms was immense.
Uranus hated his children, including the Hundred-Handers, and as soon as each was born, he imprisoned them underground, somewhere deep inside Gaia. As the Theogony describes it, Uranus bound the Hundred-Handers
brother or sister kin