According to Hesiod, before anything else, there was Chaos (Χάος). Chaos was a space which included by nature everything that would create the Universe. So Chaos created Darkness (Έρεβος, the underground space that was as deep as the Sky was high) and Night (Νύχτα). Darkness and Night brought forth Air (Αιθέρα) and Day (Ημέρα). Thus, since Chaos created Darkness and Night, he was the main and the first beginning of everything.
After him, there was Earth (Γαία), the most solid base of everything, and Eros (Έρως), the most beautiful of all immortals who rejoiced all mortals and gods. Eros later took the meaning of romantic love and in general, the power of attraction that leads all elements to a union and the creation of new ones. This power is unlimited and most powerful; nobody can escape it.
Earth was the mother of all that exist and she first gave birth to the Sky (Ουρανός) who covered her with his starry cloak and became the never ending temple of the immortals. Earth married her first born and from their union generated the first dynasty of the Gods. Those were the twelve Titans, the males were Oceanus, Coeus, Crius, Hyperion, Iapetus and the younger Cronus. The females were Theia, Phoebe, Rhea, Mnemosyne, Themis and Tethys. The Titans’ siblings were the Cyclopes, the one eyed entities, Brontes (thunderclap), Steropes (lighting) and Arges (bolt), who were full of pride and arrogance.
However, the Sky was aware that one day his sons would take over his throne therefore, he would throw them underground, deep in the earth in Tartarus. Desperate Mother Earth asked help from the rest of her children but only Cronus was brave enough to help her bring Sky down.
Cronus hid armed, waiting for the right moment to attack his father. Sky arrived with Earth, ready to make love to her, so he lays on all her width and length. Right then, Cronus left his hiding spot, immobilized his father and with his right hand cuts of the Sky’s manly part. From the blood that was spilled from this terrible wound, the Erinnyes, the Giants and the Nymphs were later born. Cronus threw the cut-off organ of his father into the sea and she kept it until white foam escaped her insides and that was how a daughter was born, who was no other than Aphrodite.
Cronus was no different than his father, though, as he too knew that he would fall from his throne by one of his sons. However, after he married his sister Rea, instead of throwing his children in Tartarus he preferred to swallow them once they were born. His daughters were Estia, Dimitra and Hera and his sons were Hades, Poseidon and later on Zeus, who was the only one to escape his stomach. That is because, Rea following the advice of their parents, Sky and Earth, saved him by escaping to Crete and giving birth to him there, while she gave Cronus to swallow a rock in a blanket.
Of course, Zeus survived and grew up in Crete were he became big and mighty to later dethrone his father. He then released from inside of him all his swallowed siblings along with the rock that was used as a substitute of himself. That was the third and last dynasty of the Greek Gods, the dynasty of the Olympic Gods.
Source : Ρίσπεν Ζ., Ελληνική Μυθολογία, Εκδόσεις Τριήρης, Τόμος 1ος, σ.11-16
 Immortals and Gods committed incest regularly as it was permitted, in contradiction to mortals for whom it was a great sin.