I don’t think Erin ever thought her sister was actually not going to let her back in the house. Andrea was probably mad too and it doesn’t sound like she tried to get a hold of Erin after she walked back to the moms. Instead, it sounds like she just tried to sleep in the broke down car at her moms. That’s rly sad.
Andrea prob felt really alone at that moment. Her one sister locked her out of the house she was living in, her other sister turned her away.... I’m not surprised she chose to sleep in the car vs calling the sister who just kicked her out. Or turned her away. However you want to phrase it.