They’re going to change in the second book. The first book was the when Skylar got kidnapped. The girls treat her like a prisoner when Skylar tries to escape or when she acts like a prisoner. It’s not that they don’t want to treat her well in the first book, it’s that Skylar is still a prisoner. She wanted to escape but also felt some affection and love to the girls because they didn’t treat her all bad 24/7. Skylar has Stockholm syndrome. That’s why you see her wanting to escape one second (because deep down she knows what’s happening to her isn’t right) and then wanting to be close to the girls and saying that she loves them the next second. She has Stockholm syndrome.
In the second book you can clearly see character development in some of the girls. They’re not hitting her as much (or really at all) even though Skylar still mouths off at them. They still yell at her and get angry but you’ll see that they’re relationship will continue to get more sisterly throughout.