In the little group, there were three rules. No trains, don't bring up Bill's mom, and don't leave Prez home alone.
The first rule was for obvious reasons. Prez had bad flashbacks and a panic attack at every railroad crossing, and Girlie would always tense up. Both of them were obviously terrified of trains, and they were to be avoided at all costs unless absolutely necessary.
The second rule was because Bill's mother had kicked him out when he was fifteen, about a week before he met the Appleshine sisters. Whenever she was mentioned, the normally goodnatured teen would darken and he would be in a bad mood the rest of the day. Bill's mother was not to be even thought about.
As for the second rule, it had to do with Prez's second fear; being alone.
It wasn't put into action until the first Valentine's Day they spent in the abandoned building. Bill had gone to get Prez something, and because she was asleep he took Girlie along - she knew her sister better than anyone else, after all.
They hadn't been gone very long - an hour or two at most. But when they got home, they found Prez curled up in the corner, sobbing violently.
Turns out she'd woken up from a nightmare about losing Girlie in a train crash, and when she woke up and searched for them only to find nothing she'd assumed the worst. It took three hours to calm her down and assure her that nobody was going anywhere.
After everything was calm again, Prez had fought tooth and nail against the rule, saying that she would be perfectly fine as long as they gave her some warning. Neither Bill nor Girlie was convinced, however.
Nobody wanted to put her through that again.
Together they would stay.