In most sources, Olympian gods were (nearly?) all directly descended from Chaos.
Were they unique in this (having a shared ancestor deity, not too far up the family tree), or was it a common thing for a polytheistic pantheon?
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Sign up to join this communityIn the Babylonian creation mythos, the Enuma Elish, the gods are created as a result of the union between two primordial entities: Apsu (male) and Tiamut (female):
When in the height heaven was not named,
And the earth beneath did not yet bear a name,
And the primeval Apsu, who begat them,
And chaos, Tiamut, the mother of them both
Their waters were mingled together,
And no field was formed, no marsh was to be seen;
When of the gods none had been called into being,
And none bore a name, and no destinies were ordained;
Then were created the gods in the midst of heaven,
Lahmu and Lahamu were called into being...
Ages increased,...
Then Ansar and Kisar were created, and over them....
Long were the days, then there came forth.....
Anu, their son,...
Ansar and Anu...
And the god Anu...
Nudimmud, whom his fathers, his begetters.....
Abounding in all wisdom,...'
He was exceeding strong...
He had no rival -
Thus were established and were... the great gods.Source: ENUMA ELISH, THE EPIC OF CREATION, L.W. King Translator
Later in the epic, Apsu wishes to kill his offsprings (for being a bit too noisy), which results in him being killed by his grandson Ea (aka Enki), who then becomes leader of the pantheon. The epic has broad similarities with Hesiod's Theogony and with the Jewish Genesis myth, and could perhaps have been an inspiration for both.
Wikipedia has a helpful diagram of the family tree of the Babylonian gods.
"in what stories are all the main characters part of the same family"
I would be downvoted. I don't think we should have myth-id questions in the first place, but if we are going to have them, I'm going to put my foot down on questions where the goal is to identify every myth with a certain characteristic.