Isis and Osiris were happily married and had a beautiful son named Horus, who would grow up to become the Patron of All Pharaohs and Ultimate Badass of the Egyptian Pantheon. Meanwhile Nephthys — sister of Isis, who will be referred to as Neffie — was not-so-happily married to Set, the Patron of Aridity, Storms, and Douchebaggery. As the renowned ultimate hero of the pantheon, Set was a macho douche-canoe who liked to throw his weight around (and undoubtedly slapped poor Neffie around whenever he felt like it), so it’s understandable that Neffie would rather bear a son for Egypt’s Sweetheart, Osiris, than for mean ol’ Set. The fact that Osiris didn’t have the head of an anteater also helped.
Isis often busied herself with her favorite hobbies: learning magic spells, grooming her pet scorpions, and blackmailing the other gods out of their shares in the “godly power stock market”. She was off engaging in one or more of these activities one evening; meanwhile Osiris was home alone, having a night cap which, as usual, had gotten out of hand. When Neffie slinked into his bedroom wearing one of her sister’s sexy form-fitting dresses and acting like a cat in heat, Osiris was too drunk to know it wasn’t Isis.
“Giveth me s’m sugar -hic- Baby,” he slurred as he collapsed face-first into her cleavage.
As a result, Neffie gave birth to Anubis, the jackal-headed Patron of Embalming Corpses For Burial, Believe It or Not. Neffie feared her husband would question why his son didn’t resemble him at all — an odd thing to worry about, since it was normal for an Egyptian god to lose the “random animal head” lottery — so she gave Anubis to a human couple who couldn’t have children, leading into a spin-off series about the weird dog-headed kid trying to adjust to normal suburban life.
Around this time, Set murdered his brother for his territory, dismembered the body, and mailed a piece to every city in Egypt, all while retaining his “perpetual Employee of the Month” facade. Given Isis’s reputation as a scheming hussy who couldn’t be trusted with a nickel, nobody believed her accusations, so she made plans for a long, dreadful journey to reassemble her hapless husband and bring him back to life. Neffie, the Patron of Awful Timing, chose this moment to bring up Anubis.
“I didn’t know you had a son!” said Isis. “Why didn’t you tell me? I coulda hosted your baby shower!”
“Ah, yeeeah, about that…” said Neffie, fidgeting, avoiding eye contact. “I kinda dressed up like you and bumped uglies with your Ozzie while he was drunk. Then I got preggoes. Then I gave birth to Anubis.”
“Oh,” said Isis.
“I’m so glad you’re taking it so well!” said Neffie when Isis didn’t immediately blast her to component atoms. “Say, while you’re on your travels all over Egypt during the next decade — y’know, reassembling Ozzie and all that — why don’t you pick up Anubis from school and take him along on your trip? Then you can get to know your stepson better!”
“Oh,” said Isis.
Thus Osiris became King of the Underworld and Guardian of the Dead, rather than explain to his terrifying super-wizard wife how he could possibly mistake someone else for her during sex, drunk OR sober.