I think deep down Isis suspected Seth's "real reasons" for giving in to Osiris's blackmail "so easily"... Awful, yes, but not completely illogical considering she had just heard Neftys' story and how even her own sister had given in to Osiris' machinations because she had secretly always been in love with him.
yeah i get she was feeling super vengeful and betrayed by everyone around her, that's why I can kinda empathize. She was also blinded by her love for osiris, despite everything pointing in his direction. I'm not sure about neph being in love with osiris, I thought she only slept with him to give seth a child and that's why she feels so guilty?
i get why isis is pissed, but instead of cursing everyone just stab osiris????? literally all this shit is his fault in the first place???