Oh, no. Mattie hadn't even thought of Nick having family in there. The gunshots made her shiver and stop and stare into the entrance wanting to go anywhere but in there. She trailed behind him at a slower speed, fear making everything just a little blurred at the edges. And what were they going to do with his mother's body? Everything was getting to be so much, and his sister–– oh, god, his sister was infected, wasn't she? 

It was getting dark out. She had no time to panic in the entrance of a Wal-Mart in the dark. Gulping, she followed Nick inside and turned sharply toward the women's clothing section. She changed out all her clothes and, feeling much more comfortable now, she sat on the floor and splashed some water on her face and hands. Now she felt a little bit cleaner. ...A little bit. 

She came back to Nick and his sister. With her possibly infected, she felt a strong need to keep watch. She looked to Nick. "Hey, um, thanks... for letting me stay here and stuff. I think you should get some rest and I'll guard us. You never know if the pricks out there will try and get in for some cheap deals." The reality was that she didn't want Nick to have to kill his sister if she became a zombie. She was also aware that he'd just lost his mother.