In S1 it shows their relationship aand they'are literally just brothers, they work together, all 4 siblings were happy, Seth was in love with Hathor, Osiris had his wife but he was in love obsessed with Seth, seeing things were there wasn't. Then osiris triked Hathor, cheated on Isis, put Seth's son in danger to blackmail him just to get him as his lover. Seth rejected Osiris but in the end yielded to save Anubis( osiris raped seth) so that thing about them having something like love was all in Osiris head
But in that flashback they're already gods, right? They lose their memories when they become gods, so I think that Osiris is talking about the time Seth was still a demi god or whatever, that was the period in time when he started to be obsessed with Seth. Or that's what I understand from all of this. I just recently started reading this story again so maybe I'm assuming wrong, sorry

I'm honestly curious to see if Osiris is just delusional or there was really something between them before this. Either way, that doesn't justify what he did or what he's doing right now in the slightest