@martha now, let's apply this theory into current situation. Osiris sent this mirror to Seth in hopes that his love+memories for Nephthys will be replaced by lu** for Osiris. But Sekhmet interferes and changes the course of the plan and makes sure that Nephthys is the one who looks in the mirror instead. So, Nephthys who was completely devoted to Seth was tricked by the mirror into believing that she was in love with Osiris instead. And her love and memories of Seth was trapped in the mirror. The Nephthys we saw Horus talking to in the mirror was just the consciousness of Nephthys who held the REAL memories of her husband

@martha confeared ain't wrong but let me break it down for you in more detail.
There are two mirrors (let's say A and B). The mirror A will be given to the victim by the offender who keeps the mirror B with themself. If the victim was to look at mirror A while the offender is facing the mirror B, the victim would be filled with lu** for the offender. And if the victim already has someone they love, the part of their consciousness that loves this person will be trapped in the mirror and these memories will be replaced by the new ones by the mirror to manipulate the victim to believe that they are in love with the offender instead