Well.. uh we all know how obsessed Osiris is with Seth right? So he uses a mirror which is sekhmet's property to make Seth fall in love. The mirror's powers include trapping a person inside it's infinite void if the person doesn't love the character that's reflected in the mirror. There's a another mirror that does the same which is hathor's and she gifted it to horus to make him love her.
Now the mirror that was with Seth wasn't used by him as intended by Osiris but nephyths and she saw Osiris face in it but she loves Seth so the part of her that loves Seth got trapped inside and outside nephyths is wiped clean of her true self and she engages in affair with Osiris. Horus on the other hand saw Seth in the mirror so he wasn't trapped because he originally loved Seth so there was no need of the mirror to split his emotions hence he was able to leave freely. But another property of this mirror is amplifying your deepest desire.. nephyths desire was to have Seth's child so she agreed to bearing Osiris seed and for horus you know what he did ... raping Seth n all.. which he'd never do willingly :,(
That's the thing. I hope I was able to clear your doubt. If u wish to know anything else feel free to ask
Heyyy so I had some other convo in this and I remembered that when hator gifted horus the mirror she was looking into it's counterpart from other side hoping to get her reflection in the one in horus hand but remember Seth snatched her mirror ?? Lmaooo in that exact moment his reflection insd of hathor was seen and horus luckily saw Seth in that moment. I believe for Osiris it was same. He might have been waiting on other side for Seth to pick up the mirror n look him in it but nephyths took it lmao. Or maybe he intended for nephyths to do it as a plot point of his grand scheme

Someone please explain me what happened.. My Mind cant process