Poseidon had five other siblings: Zeus, Hades, Hera, Demeter, and Hestia. Poseidon eventually married the marine goddess Amphitrite, and together they had the fish-tailed god Triton.
Poseidon was generally portrayed as one of the most mercurial of the gods. One would assume that might reflect the mercurial nature of the ocean. He generally didn't get along with his siblings, Zeus in particular, who was generally considered the most powerful of the gods. He was also jealous of Athena, who bested him in a contest to become the patron god of the city now called Athens. The gods were asked to show their power. Poseidon wanted to show his benevolence so he struck the Acropolis and up sprang a salt water stream. Athena, however, created an olive tree, which was much more useful and therefore she won the right to become the patron god of Athens.
Poseidon often had a retinue of Nereids, the fifty daughters of Nereus, the Old Man of the Sea. They were water nymphs who often appeared to help stranded sailors. The best known Nereid is Thetis, the mother of Achilles.