In the beginning there was Chaos and Gaia (the Earth) sprang from it.  She then had Ouranos (the Sky), Pontus (the Sea) and Tartaros (the Underworld) by herself, without a mate.  Gaia had several sea divinities by Pontus and some by Tartaros, but she took Ouranos as her husband due to the fact that he could embrace her fully (the ancient Greeks imagined the earth as a flat disk and the sky as a dome that was decorated with stars and sat on top of the earth).  Subsequently, Ouranos ruled the universe and, of course he and Gaia had offspring: the Titans, the Cyclopes and the Ekatonheires.  

To continue his rule, Ouranos locked the Cyclopes and the Ekatonheires in the depths of the Earth, causing Gaia immense pain.  In order to avenge him, Gaia ordered her six Titan sons (there were also six Titan daughters) to overthrow their father as king of the world and free their other brothers. As a result, one night when Ouranos lay in bed ready to sleep, four of the six Titans held him down on the four corners of the world while a fifth Titan (Kronos) castrated him.  The blood from his castration spilled onto the earth, producing the Erinyes (which were spirits that haunted those who felt guilt) and the Gigantes.  Some myths say that the goddess Aphrodite also sprung from the foam of the sea (aphros) after drops of his blood were spilled into the sea near Cyprus. Kronos was now in control of the universe  and although he initially freed his other six brothers (the Cyclopes and Ekatonheires), he ended up imprisoning them again in the Tartara to eliminate competition.  

The five siblings are:  Poseidon, Hades, Hera (who became Dias' wife), Estia, Dimitra.  All of them later ruled the kosmos on Mount Olympus with Dias as the King of the gods, but his happened after the Titanomachy (War of the Titans).