In Shirley Jackson's We Have Always Lived in the Castle, the characters of Constance and Mary Catherine (Merricat) are both complicated. At the start of the novel, they've been fending for themselves over the six years since the rest of their family died. Their Uncle Julian is not fit to take care of himself, leaving the burden on Constance to be the caretaker for both Merricat and Julian.
Based on everything that occurs over the course of the novel, it's not a stretch to say that Constance has failed Merricat and, arguably, Julian (though to a lesser extent). One could argue that Constance fails Merricat by never filling the void Merricat's parents left. There is no doubt that Constance cares for Merricat as an older sister should. However, she fails to provide the kind of structure that a teenager like Merricat might require while also failing to put Merricat's needs ahead of her own.
In fairness to Constance, she must deal with a village that despises her: they believe that she poisoned her parents and got away with it. However, Constance never confronts that issue; instead, she allows Merricat to absorb the anger of the villagers every time she is sent on errands in town. Constance is also unable to prevent Merricat from continuing to believe in obvious delusions, such as believing she's on the moon.
One can argue that Constance's failure in these areas help shape into Merricat into the type of person she is at the start of the novel. When Cousin Charles arrives at the family home, Merricat's personality clashes with Charles's, creating the conflict that fuels the rest of the novel.