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This is just in keeping with Egyptian norms of the time. In nobility, specifically, to keep the line pure and to protect their territory from foreign interests, they would marry within the family and honestly, it seemed like anything went: fathers married daughters, sisters married brothers, nieces married uncles. I think king Tut's mother was a granddaughter to the father.
Also, I just found this webtoon myself, so I need to spend some time in it and Egyptian Mythology, but I think (I may be wrong) that some of the pairing are hinted at or could be interpreted as shown here from the mythology itself. I think the Horus and Seth pairing is suggested in the myth as well while they are trying to outdo each other in battle.
The incest is in keeping with the society of the time and place the story is from.
The Osiris and Isis things you are seeing will be explained and make lots of sense later.
A hint: Pay special attention to flashbacks of a very different Seth - showing how he used to be.
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Im aware of the incest stuff that is happening but I expected the couple to be Anubis and Horus tbh. I’m a bit disappointed that they weren’t but I’m still gonna continue tho and see how they will end up. I have already seen Seth’s face and he doesn’t look like str8 like plsss...how she having his way with Nepthys but anyway im still looking forward to it tho
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I think they are just going with some of the things that are in the actual myth and creating a story around it.
While reading this webtoon, I found out that apparently something happens between Seth and Horus in the myth.
As for Seth, Anubis has a flashback on Chapter 5 showing who Seth was. He was the same with his wife. Please do continue because even though it will take awhile to build up it will be worth it.
I though the pairing or the couple would be between Horus and Anubis because it’s more fitting since their the last line in the fam tree but having an affair with your uncle is on another level ummm...seeing seth pursuing isis as well, while having nepthys as his wife and having a son plus ending up with Horus....so .....yuh
I also though Osiris has a thing for Seth seeing in one of the chapters where he takes off Seth’s mask u and like leaning in to do sumn sumn...so