Cronus and Rhea are two of the Elder Titans. Cronus, Titan of time and Rhea, Titaness of motherhood and fertility got married and ended up having six children. These children would come to be the core Olympian gods.
Cronus and Rhea both came from Uranus and Gaea, and while they call each other brother and sister, it's important to remember that they aren't actually related. The best ways I have to explain this go as follows, if you have read/seen Twilight then continue reading, otherwise skip to my second explanation.
1) In Twilight, Carlisle (the father vampire) adopted Edward, Rosalie, Emmett, Alice, and Jasper. Rose/Emmett and Alice/Jasper end up pairing off and becoming couples. They all call Carlisle and Esme their parents, but none of them are actually related. It is helpful to think of the people in Greek mythology this way.
2) It would be the same as if I were to pick 12 people off the street and tell them they are my kids. I bring them home and we all live in one big mansion together. Eventually they all pair off and pick 6 random people from another street and tell them they're their kids. No one is actually related, but we're all pretending to be. Yes these Titans and Gods came from the same beings, but it's like they are completely different beings. If you looked at their DNA, it would not match one another's.
So Cronus, with the help of Gaea, overthrows Uranus as king. Cronus takes the title for himself and rules over the Titans. Things are great until he gets a prophecy from Gaea's Oracle that says one of his children would overthrow him. Cronus couldn't bear the thought, so he decided to do something about it. Rhea welcomed their first child, a glowing little girl named Hestia, into the world and what did Cronus do? He ate her of course. Yup, that's right. He swallowed her like a baby sized pill. The pattern repeated until Cronus had swallowed 5 of their children.
Rhea was very upset about this and she sought the help of Gaea who gave her an idea. Rhea gave birth to her 6th child, Zeus, in secrecy and left him to be raised by Gaea. Rhea gave Cronus a rock wrapped in a blanket which he swallowed. He didn't know the difference. When Zeus grew to be an adult, he found his father and gave him an elixir that Gaea had made which caused him to spit up his other children. All 5 of them were fully grown at this point.
The 5 of them decided to stand by Zeus as he declared war on the Titans. Let's not forget, while Cronus is over here eating his children, the other Titans are just going about their daily business and turning a blind eye to him. This obviously doesn't sit well with the 5 people who were eaten alive and spent their first years on Earth inside of someone's stomach.
Zeus and his siblings went to Mount Olympus to set up camp and prepare for war.
If you haven't figured it out by now, here is the list of Cronus & Rhea's children:
- Zeus
- Poseidon
- Hades
- Hera
- Demeter
- Hestia
