In what ways has Holling tried to bring peace to his world

Probably the best place to look for support on how Holling tries to bring peace to his world is to look at the relationship between Holling and his sister, Heather. Through most of the book, Heather is quite antagonistic to Holling.  n her defense, she is antagonistic to just about everybody. I have to give Holling a lot of credit because no matter how mean or snarky Heather is to him, Holling never reciprocates it back to her. He wants to have a good relationship with his sister. He needs it actually because deep down Holling knows that he'll never have a great relationship with either of his parents.  
Holling begins to thaw Heather's icy exterior and harsh attitude toward him when he saves her from being hit by the bus. His actions were more reflex than conscious thought to save her and their relationship, but his actions do give Heather a wake up call about the fact that her brother loves her deeply enough to risk his own life. 
Heather will eventually run away from home, and she will get stranded far away. It isn't Mr. or Mrs. Hoodhood that rescues her. It's Holling. His father actually refuses to help, and Holling takes the situation into his own hands. He wants his sister back, and he's willing to do whatever it takes to get her back. The book ends with their warring relationship finally at peace. 

"Holling," she said. "I was so afraid I wouldn't find you."
"I was standing right here, Heather," I said.  "I'll always be standing right here."