What is the relation ship between Holling and his sister?

Holling's relationship with his sister, Heather, is dynamic; it changes by the end of the story. When readers are first introduced to Heather, it is clear that there is quite a bit of antagonism between her and Holling. For example, the first time that Holling speaks to his sister, she tells him that she hates his guts:

"Mrs. Baker hates my guts," I told her. 
"So do I," she said.
"I could use some help with this." 
"Ask Mom." 

It is clear from this exchange that Heather and Holling are not best friends with each other. They really are not even on general speaking terms with each other. That does not mean they do not love each other though. In fact, Holling risks his life to save Heather from a skidding bus. His love for his sister continues to show throughout the rest of the story. It is Holling that pays for her to return home after she runs away and gets stuck. She knows better than to talk to her dad or mom about it, but she knows that Holling loves her and she loves him. When she finally returns home, she breaks her icy shell and hugs Holling. It is her way of saying "thank you" and "sorry" at the same time.

They did not start up again until my sister got off the bus, and she ran out of the diesel combustion and right to me, and we held each other, and we were not empty at all.  
"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."