I wanted to respond to a few things you pointed out.
Cassie did not write about Sarah feeling sympathy, guilt, or remorse. That doesn't mean Sarah didn't feel those things, but many times, anger and resentment are easier emotions to handle and like Thorn pointed out in another comment, victims tend to defend their abusers. Perhaps the blame for this is on me, because I did not explore her feelings enough near the end.
I did not mean to indicate that Grant has had a problem setting boundaries with Sarah for his entire life. Elliot and Grant set a perfectly reasonable boundary. Don't tell your crazy boyfriend where you are staying and obviously don't have your crazy boyfriend over to our house. She chose to violate that boundary to continue to see her abuser. I understand why she did it, but she was still in the wrong. Therefore, Elliot was 100% in the right not to allow her to stay there. Grant was choosing to prioritize his sister's safety, also understandable, but by allowing her stay in their home again, he was also violating that boundary based on past behavior. Elliot is quite good at setting boundaries. Grant, not so much, and definitely not where his sister is concerned. If Grant and his sister do reconcile, setting boundaries is something he'll have to learn.
Sarah's behavior was very frustrating, which seems to be a common sentiment with people who have loved ones in unhealthy relationships, but I would put most of the blame on Jim for being a horrible, abusive, and manipulative monster.