It'll explain further in the next chapter. But basically
Nephthys really did love Seth, and she wanted to "tie" hi down with a child and having a child was her BIGGEST desire. She goes to Osiris asking for guidance or something, but ends up having the spell of the mirror work against her.
Since earlier she saw Osiris in the mirror and both Sekhmet's and Hathor's eyes of the mirror opened, it trapped her memory love for Seth inside because she saw someone she doesn't love.
Unlike with Horus, he saw the person he loved and the spell for him to fall in love with a different individual (I.e the other person in the mirror, was nulfied since he saw Seth.)
So Nephthys under the spell, was made to have her desire be timed by 1000%. So since the mirror made her fall for Osiris, she asked him for his seed because her desire was to have a child.
So yeah. That's how the mirror works.
Remember though that this was osiris' doing. Sekhmet just used the tools around her to tweek the moment of what and who is in front of the mirror. Sekhmet didn't want Osiris to get away with stealing the mirror. So she ruined his plan to have Seth be the one to look in the mirror and had Nephthys look instead.
Someone please explain what happens Nemphys and the mirror. I’m so confused…