Sadie and Carter stay in Brooklyn with Amos after the red pyramid is destroyed. Mostly they're trying to fix up the mansion, but they're also trying to fix Amos, who was pretty broken after Set got through with him. The kids have a rendezvous with their parents in the Land of the Dead, and they see Horus crowned as king of the gods.
But wait—there's more. Bast returns from the Duat to take care of the kids, so there's much rejoicing. Then Sadie has a vision about a school where there might be other descendants of the pharaohs, who may also be in danger. So the kids and Bast go on a road trip to store the amulet of Osiris in the school in order to clue in the newcomers to the danger they're in—and to the power they can access.
Carter and Sadie also finish recording the narrative of their adventures, and they're arranging to have it published so it can fall into the right hands (which might, in fact, be your hands). The kids are super aware that there's more that's going to happen: they'll see their parents in the underworld again, which means that Sadie will get to see Anubis again (insert kissy noises here). Carter, of course, plans to look for Zia (more kissy noises). Most of all, they're preparing for chaos to rise. It's going to be quite a fight.
As Carter sums it up: "Most of all, chaos is rising. Apophis is gaining strength. Which means we have to gain strength too—gods and men, united like in olden times. It's the only way the world won't be destroyed" (41.209). Cheery, right? But it's heartening to hear Carter sounding so determined, because in the beginning of the book, he was kind of wishy-washy about the whole "spend time with your family" thing.
What's the significance of this particular ending? We're so glad you asked. Remember at the beginning of the book, when Sadie and Carter could hardly stand each other's company? They were annoyed by one another, and they felt like almost total strangers. Yeah, that changed over the course of the book, to the point where they not only work as a team—"the Kane family has a lot of work to do," as Carter says (41.210)—but also want to work together to reopen the path of the gods.