Mum had always been strange, had never liked Nick, but it was Dad's job to take care of Mum, just like it was Alan's job to take care of Nick. (3.3)
In all families, whether they're functional or dysfunctional, people tend to take on certain roles, and the Ryveses are no exception. Early on Dad protects Mum, and Alan protects Nick. And when Dad's gone? Well the jobs shift a little over time, but the Ryveses remain stuck together by family ties—even though none of them are actually blood relatives and two of the family members can't stand one another. Family can be strange.
"[…] I know you're worried," Alan said. "Don't be. How many people with first marks have we seen? How many first marks have you removed? How is this different?"
Nick turned his gaze from the window to Alan.
"This is different," he said. "This is you." (3.50 – 52)
Ever heard the phrase close to home? That's why Nick's particularly troubled by Alan's mark—because it's hitting close to home. We rarely worry about things so much as when they begin to affect people we know and love, people with whom we share a home, people we consider family.
Alan had reached out and smoothed Nick's rumpled hair. "You're mine," he said, in a trembling young voice that already had a ring of Dad's about it. "And I'm going to take care of you." (6.157)
After Daniel Ryves dies, Alan, at the ripe old age of eleven, becomes the man of the house, and it's at this time, shortly after his father's death, that he utters these words to Nick. Clearly the importance of family and his obligation to the other members of his household was impressed upon Alan early, and you know what? That's actually another characteristic that's common in dysfunctional families: children who play parental roles from an early age.
He'd carry out Alan's little plan, even if he was in the dark. This was his brother. He had no one else. (6.194)
Yeah, sometimes families inspire leaps of faith like this—especially when family is all you have, or all you think you have. Is Nick right? Is Alan really all he has? Why or why not?
Mae would probably be better at comforting Alan than he would. Jamie would probably be better at it. Nick did not have the slightest idea what to do, but Alan was his brother and no one else's, and he would think of something. (10.154)
This is one of several occasions in which Nick is willing to step outside his comfort zone or tolerate something he normally wouldn't just for his brother. Their brotherly bond tie is a strong one.
"I'm not stupid," [Mae] said. "I'm attracted to you, I could be attracted to Alan, but what does it matter? […] I won't let myself be used, and I won't let whatever crisis you're having hurt my brother's chance to live." (12.77)
Even if Arthur had been hunting them for Mum's charm all this time, he might not want Nick killed. He was Arthur's son.
He could use that. (13.130-31)
At one point Nick claims that he doesn't understand human love, but we're not so sure that's true because here, at least, he demonstrates that he understands something fairly similar: the ties that bind family together. Unfortunately, Black Arthur doesn't.
The spell of blood to blood sang in Nick's veins, as if there was a kettle somewhere bringing his blood to a boil. For a moment […] all he could feel was the tug of connection between himself and this man, the link formed of shared blood. (14.8)
Okay so Nick and Black Arthur share a bloodline. Is that enough to make them family, or is something more required? What is it that bonds people together as family? What requirements have to be met? Is shared blood one of them?
Arthur only wanted what he was owed: what Nick had promised him.
He hadn't promised him Alan. Black Arthur had no right to touch his brother. (15.94-95)
Silly Black Arthur—he thought that he could taunt Alan, shove him even, without upsetting Nick. He thought Nick was incapable of feeling love or loyalty for the brother who'd helped to raise him. He thought Nick had no concept of family. He thought wrong.
[…] in the gathering quiet it occurred to him that he had left Alan in a nest of magicians.
He went back faster than sound or light, fled uncaring through a city he could have owned to the one place that mattered […]. (17.31-32)
See that? The "one place that mattered" is the place where his brother is. It's kind of like that saying—home is where the heart is. For Nick, his brother = heart, and heart = home.