What are some other important family relationships in the story?

Byron and Kenny: Kenny pretty much idolizes his older brother. In chapter 2, he notes,

Even though my older brother was Clark Elementary School’s god that didn’t mean I never got teased or beat up at all. I still had to fight a lot and still got called Cockeye Kenny. . . It seemed like one of these things happened to me every day, but if it hadn’t been for Byron I knew they’d have happened a whole lot more.

For most of the novel, Byron torments his younger brother in the ways only big brothers can, but after the church bombing in the end, Byron is the one who talks to Kenny about racism, police brutality, and the unfairness of the world.
Kenny and Joey: Joey is the baby sister who is almost too good to be true. When Momma tries to punish Byron by burning him, little Joey blows out all the candles. When she is nearly one of the girls who dies in the church bombing, Kenny is overcome with fear and grief. In talking about this, Kenny tells Byron:

But Byron, it’s just not fair. What about those other kids, you know they had brothers and sisters and mommas and daddies who loved them just as much as we love Joey, how come no one came and got them out of that church? How’s it fair? How come their relatives couldn’t come and warn them?

Part of the reason this relationship is developed is to help the reader see that each of the four girls who perished in the real-life bombing was someone else's little Joey; each girl had a family who loved her.
Kenny and Grandma Sands: Although Grandma Sands is a minor character and they mostly only know stories about her, she still shapes their childhoods, even before they go visit her. Grandma Sands both demands respect and showers the kids with love. When Kenny's mother (Grandma's daughter) questions her about a certain gentleman, Grandma Sands puts her in her place, and Kenny notes:

Grandma Sands didn’t yell or scream or anything, but the way she said those couple of things made everybody who heard it shut their mouths and listen real hard.

Grandma Sands provides a firm yet loving role model that Kenny will need after the church bombing.
Kenny is surrounded with a diverse group of supportive relationships, and that's part of the reason he is able to recover from witnessing the tragedy that shook him to his core.