After the war between the Olympians and the Titans, the Universe was divided among three males. These males were Poseidon, the God of the Sea, Hades, the God of the Underworld, and Zeus, the God of the Earth and Sky and the Chief God.
Zeus married Hera, the Goddess of Marriage, and moved to Mt. Olympus. Mt. Olympus is a real mountain in Greece that is over 9000 ft. tall. It's the perfect place for a god.
Zeus and Hera had children and Zeus had children with other women and, believe it or not, Zeus had a child by himself.
So in other words, all the gods on Mt. Olympus are Zeus's children.