"He always comes over on Sunday," I said. "He expects me." (6.106)
Dominick is a man of his word, and even though he could write Thomas off as being crazy, he still wants his brother to be able to count on him.
"I've always been the one who had to look out for him. Not that I ever signed up for the job, believe me." (7.94)
Despite making every effort to be loyal to his brother, Dominick isn't happy about it. He feels more like it's a burden than happy to be doing his brotherly duty.
"Could you just do me a favor and be his roommate?" (8.21)
Dominick wasn't always loyal to Thomas. He'd do anything to get out of being Thomas's roommate in college, but his Ma talks him into it—showing, again, where her loyalty lies.
Yeah, I'd type the stupid paper, I told him. (8.57)
Sometimes Dominick's loyalty to Thomas borders on enabling. By doing Thomas's work for him, he makes it easier for Thomas to slack off later.
I feel bad for him. And mad. And humiliated. Kids are looking at me, too, not just at Thomas. (10.45)
Dominick might stand up for Thomas, but he hates being associated with him. He's embarrassed by him. But in this case, maybe the other kids just don't know which twin is which… not like that would be a good thing for Dominick's ego, either.
"Winning the trust of someone suffering from paranoid schizophrenia is a long, slow process. […] And a tenuous one. A rickety bridge." (15.23)
Trust and loyalty is important when it comes to building a relationship with a person who has schizophrenia. They are naturally paranoid and wary, so the slightest betrayal can be the end of the world for them.
"Why don't you worry about yourself for once instead of your brother?" (30.138)
Joy might not be the most loyal partner (she cheats on Dominick, among other things), but in this way, Joy is loyal to him: She doesn't want him to take care of Thomas at the expense of himself.
"My mother. I promised her I'd take care of him." (36.11)
Perhaps Dominick is less loyal to Thomas than he is to his mother and the promise he made her to take care of Thomas when she died. Do you think this matters?
When we pulled into the parking lot at McDonald's, who pulls in right behind us, honking, but Leo. (40.202)
We can't think of many good qualities Leo has, but loyalty is one of them. He's always nearby to support Dominick, and therefore Thomas, whenever they need him; they don't even have to ask.
"Good. Because you and I are on a team, all right? We're buddies, you and me. We stick together. Right?" (42.150)
This sounds sweet, but this is Ray talking to Dominick while he's abusing Thomas. Dominick does not want to be a part of this team, yet he is. The fact that Ray is loyal to Dominick is something that bothers him for many years.