One of the things that makes the Theogony so weird is that it's full of characters that don't actually do anything besides exist. Cronus' brother and sister Titans fall into this category. Hesiod tells us when they were born, who they married, and who their children are, but these guys (and gals) don't actually do anything during the war. Still, we'll list what we know about them here, just for reference. Oh, and remember, all of the original Titans were born to Gaia and Uranus.
Oceanus: also known as the "world-sea," Oceanus represents the ocean. He is married to Tethys, and is the father of the Oceanids, who represent the world's rivers, streams, lakes, etc.
Tethys: goddess of the sea, wife of Oceanus, and mother of the Oceanids.
Hyperion: also known as the "lord of light," Hyperion is the father of Helios, Selene, and Eos, who represents the sun, moon, and dawn. In some versions of mythology, Hyperion is also the guardian of the East.
Theia: wife of Hyperion and mother to Helios, Selene, and Eos. You'll notice that the female Titans don't have much of a role other than being married to someone. This partially reflects the not-so-cool culture of the time.
Coeus: guardian of the North and father of Leto. Leto sleeps with Zeus later on, and the two of them give birth to Artemis and Apollo.
Phoebe: wife of Coeus and mother of Leto. Through Leto, Phoebe is the grandmother of Apollo and Artemis; this might be why Apollo is sometimes called Phobos Apollo.
Rhea: sometimes thought of as "the mother of Gods," Rhea is often associated with Gaia and the earth. She is the wife of Cronus, and the mother of the Olympian Gods: Zeus, Hera, Hades, Poseidon, Demeter, and Hestia. She begs Gaia to save Zeus from Cronus, and then tricks Cronus into eating a rock.
Mnemosyne: goddess of memory and mother to the nine Muses, who inspire creativity in mortals. Most traditional ancient Greek poetry opens with the author sucking up to the Muses. In the opening of the Theogony, Hesiod explains that the Muses visited him and asked him to tell the story of the creation.
Themis: goddess of divine law and justice, and the founder of the Oracle at Delphi, where many Greek heroes went to have their fortunes revealed. Themis was herself Oracular, meaning she could see the future, and some versions of mythology claim that she is the mother of the Moirai, the Fates.
Crius: this guy did nothing. Don't pay any attention to him.
Iapetus: also known as the "god of mortal life," Iapetus is the father of Atlas, Prometheus, and Epimetheus. Prometheus is the god who stole fire from Heaven and gave it to mankind, and so Iapetus is considered one of mankind's ancestors.