Who are poseidens brothers?

Poseidon's brothers were Hades and Zeus. Hades was the god of the underworld and Zeus is the leader of the gods.
Poseidon also had 3 sisters, Demeter who is the goddess of grain and fertility, the pure. Nourisher of the youth and the green earth, the health-giving cycle of life and death, and preserver of marriage and the sacred law, Hera goddess of women and marriage and Hestia goddess of the hearth.
Poseidon also had a son with Thoosa a Nereid (a sea nymph) called Polyphemus who was a cyclops (a giant with only one eye on their forehead), Polyphemus got his eye stabbed out by Odysseus.