Idk but I see two possibilities.
First being that Seth and Osiris were actually lovers (which fucking disgusts me) and then Seth's memory got erased and he forgot about everything.
Or second, Osiris was always obsessed asshole and thought (or forced Seth) they were in a relationship.
Then theres HTE who's still soo mysterious, even after we know about his existence for some time now, and i can actually see him and Seth being lovers before, buuut I think that would be a bit too much of a cliché.
If u refer to that one mural hieroglyphic translation that lazuli Lotus made abt the infinite void cave chapter theres quite some cryptic hints abt their past lives. Abt a brother who didnt like war and was very inquisitive, a brother who followed his elder sibling everywhere and an elder brother who taught his younger brother and soon grew very fond of him. Now as u said osiset being past lovers is surely a vague probability but a situation where osiris teaches younger brother seth and soon becomes obssed with him doesnt sound too far fetched either. I can pair this with a few more instances
I just know when everyone involved recovers their past memories, this gonna hurt even more. Osiris is so delulu thinking past Seth was his. He must have done something to his own memories because no way Seth loved him unless he was threatened, to protect someone or in despair. Never willingly. Especially because of HTE.
And Hathor this, Hathor that, everyone who care about Hathor really did everything except told her the truth since the beginning.
So close to the season ending.