
Isis is not using her brain atm gosh she is being such a fucking pathetic Bitch, she could not bring herself to kill/curse Osiris so she put all her anger upon Seth like bruh fuck you and your stupid mf husband :<

She did curse Osiris too! Isis cursed all four of them, herself included, to “not getting what they want the most”. Nephtys won’t have the death she craves, Seth won’t have Anubis (his family), Osiris won’t have Seth and Isis won’t have Osiris.
Despite knowing the truth and witnessing her husband blackmailing and raping their brother, she’s also furious with Seth because he didn’t go to her for help and on the contrary, he tried to force her to marry him, turned violent and despotic, and obsessively chased her and baby Horus slaughtering thousands of humans in the process. She was abandoned by the other gods, who blamed her for Seth’s rampage, and forced to live among the lowest of low.
That being said I don’t justify what she’s planning to do and I still do not like her, but she has reasons for being bitter at the very least.

Isis didn't kill Osiris because no one can kill Osiris, he's the god that can't die unless HE HIMSELF agrees to it (we'll see how Seth cleverly managed to deceive Osiris in order to get him to agree to his own death -so that he could be rid of him- in just a few more chapters). And like the above reply said, Isis cursed him too.
The fact that Osiris can't die unless he agrees to it is supposed to be what made Osiris replace Ra as king of Egypt, as he was even impossible for Ra to defeat.
RouRou is such an angel, how I wish every child was as kind and soft as him though no one can replace my cutie Rourou