People know Chuck and Amara can still be twins while Amara is older right. My dad is a twin but he's also the oldest of my grandma's kids because he was born before his twin. Chuck and Amara being twins doesn't mean one can't be older than the other and it makes sense for darkness to be first because the universe started with darkness. This is even mentioned in Genesis (although it doesn't apply the same because in Supernatural, Chuck is the embodiment of light and Amara is the embodiment of dark). Additionally, the Big Bang was an act of destruction that resulted in creation, going by Stephen Hawking's theory about it — a single atom split (was destroyed) to bring the universe into existence (create it).
So it makes sense for Chuck and Amara to be twins — it also makes sense for Amara to be the older of the two. Those two ideas can coexist.















