I have several theories. It coud be because of any or a couple of these reasons:
1) She may have figured out Osiris would always find ways to use Neftys' deceit to undermine and subdue Seth's self confidence and god powers, as long as Seth stayed "tied" to Neftys.
2) She may have figured out Osiris would always find ways to use Anubis to blackmail, threaten and manipukate Seth, as long as Seth stayed "tied" to Anubis.
3) She may have finally (and stupidly) bought into Osiris' bull*** insistence that if Seth hadn't one-sidedly pressured Neftys so much to give him children, Neftys wouldn't have restorted to desperate measures and given so easily into Osiris' own machinations, and thus they may not have fallen in this "irreversible" mess ("irreversible" meaning Anubis' being Osiris' son as something irreversible).
4) She may have (wrongly) suspected that Seth, like Neftys, also had "secret reasons" for giving in to Osiris "so easily"
ME WANT TO KNOW TO WHY SHE CURSE SETH????
Tyson it's was really interesting to read
Jealousy. Because Osiris loves Seth but she loves Osiris. If she cant have what she want, nobody will neither.
Why did Isis curse Seth?