Losing their daughter Ruby must have been rough on the Fausts. Raising Sadie must have been difficult too, since she's Little Miss Rebel. They're not major players in the book, but here's what we know about them. Sadie describes Gran as "frail and colorless, like a stick person really" (4.8). We'll chalk it up to grief.
In contrast, "Gramps is a former rugby player. He has beefy arms, a belly much too big for his shirt, and eyes sunk deep in his face, as if someone had punched them…Gramps is quite scary looking" (4.10). We get the sense that his bark is bigger than his bite, though, as he never does actually beat up anyone in the novel.
After the explosion at the British Museum and the news that Sadie must go away, the Fausts react according to their personalities. Gramps is so angry when Amos shows up for the kids that he says: "If I was younger, I would beat you to a pulp" (4.46). Gran, on the other hand, is "trembling" when she protests that Amos can't take Sadie due to their agreement (4.58).
However protective they feel about Sadie, that same emotion does not extend to Carter: "He knew that they didn't want him around. He'd always reminded them of our dad. And yes, it was a stupid reason not to take in your grandson, but there you are" (4.79). We're guessing that Gran and Gramps aren't actually bad people (despite how violent Gramps seems to be), but losing your only child can tear apart your family. At least they provided a good home for Sadie for a while.
We should mention, by the way, that the name Faust reminds us of the character Faust (sometimes spelled Faustus), who is connected with magic and knowledge, and not always in a totally positive way. The name Faust conjures up some doubts about whether knowledge and magic are always good things—and as we see in this book, both of those things can be used for evil as well as good purposes.