Via describes her family as a universe in which August is the sun, with everyone else revolving around him. While the metaphor aptly describes the mechanics of their family (they do all generally fret over August), it also suggests Via's larger sense of powerlessness—the universe is fixed, the system is unchangeable. Just look at the epigraph for her section of the book:
Planet Earth is blue and there's nothing I can do.
But there's something else that's important to think about with Via's universe metaphor. While it's easy to assume that everyone orbiting around Auggie is just a bad thing, the comparison of him to the sun also suggests that he's the main source of light in their lives—or at least in Via's life, since this is her metaphor we're hanging out with. And since the sun illuminates the world, we can also see Via acknowledging that her understanding of the world around her is illuminated by having Auggie as her brother. In short, this symbol—like family—is complicated.
But wait. Check out this other thing that Via says too:
But this year there seems to be a shift in the cosmos. The galaxy is changing. Planets are falling out of alignment. (2.A Tour of the Galaxy.5)
So while it all might feel larger than life for Via, she can also sense that it's going to be a big year for the Pullman family. Things may have been following one course for quite a while now, but she's off to high school and Auggie's ending his time as a home schooled kid, and that means it's time to reorient. So long as the Pullman's stick together, we think they'll find their way just fine.