
Im confused as well, but I think Anubis' problem is that he doesn't have a body that can allow him yo turn into a God since Osiris destroyed his original body, so his current temporary body is being consumed by the spirits that make Seth's sword, which he used to cut Osiris' arm as well as Anubis's since he possessed his body. About the curse, I think it's so that none of them get what they crave, Osiris craves Seth so he will never fully have him, Isis craves revenge and Seth craves the his son's safety and for him to have a real body. That's honestly all I understood, I definitely need to read this a second time.

Anubis currently has two curses on him. The first curse's been on him since before the beginning of the story, though he and the rest all found out about it only recently and it's still unknown who cast it on him, and it's the reason he's been stuck as a kid for centuries, unable to grow up to an adult and unable to become a god. The second curse is the one that Seth intended for Osiris but accidentally gave to Anubis instead, when he tried to cut off Osiris' arm only to find out he had just cut off the possessed Anubis' arm instead. That second one is was made with the resentful souls of the people Seth killed, and it's a lethal curse that will eventually kill Anubis unless they manage to find a way to overcome it before that happens.
Ra, you sexy beast of a woman, take my heart!!!!!!!!