Ohh so Nephthys' real feelings are that she loved Seth. But the mirror basically manipulated her into believing it was OSIRIS that she loved. Cuz the mirror makes you fall in love with the person that it reflects it to.
Remember when Horus picked the mirror up? If it was harthor that was holding the mirror and looking into it, the memories of Seth and the love he had for Seth would have been swapped for Harthor, but since Set was holding it while teasing Harthor, nothing happened for Horus cuz the mirror was reflecting the person he already loved.
But when Nephthys picked up the mirror, the other side of the mirror/the second mirror (there are two mirrors guys..) It didn't reflect the man she currently loved, instead, it reflected Osiris, which caused her feelings for seth swap for Osiris.
Idk I have a feeling that Osiris wanted to use that mirror for Seth, to make him fall in love with him.