The Titaness who married her brother Hyperion and became the mother of Helios, Selene, and Eos was Theia.
Theia was one of the children of Gaia, Earth, and Uranus, Sky, and therefore belonged to the older generation of divine beings known as the Titans. In the standard genealogy, she became the consort of her brother Hyperion. Their children represented three major heavenly lights: Helios was the Sun, Selene the Moon, and Eos the Dawn.
Theia’s own mythology is relatively sparse, but her name and epithets connect her with sight, brightness, and shining brilliance. She was also called Euryphaessa, a name associated with wide radiance. Ancient poets used her genealogy to explain how the celestial lights entered the divine family tree.
Some ancient accounts vary. Homeric and later traditions sometimes use Hyperion as an epithet for Helios, while Diodorus Siculus preserves an unusual story in which a figure called Basileia marries Hyperion. Those variations do not displace Theia as the canonical answer to this genealogy question.