Could that have been the reason she kept them isolated? Some women who keep babies conceived in rape resent them and abuse them. Violet may have seen her daughters as constant reminders of what that horrible man did to her, so she enacted a sort of vicarious revenge on him.
Granted, she seemed to be a good enough mother that Nell lovingly placed flowers on her dead body. But perhaps their relationship was more complex than that.
I thought it was clear that Violet saw the girls as innocents ("good child") and wanted to protect them. The key is Violet's frequently quoted verse from Isaiah about the sinful nation with all the evil-doers. She told the girls that's what the world was like. She may have scared them with that stuff, but she didn't resent them. If anything, she saw them as a blessing.
You've got me?! Who's got you?!
I just went back to the FAQ section to look up the exact Bible verses Jerry reads to Nell:
Isaiah 1:4: A sinful nation, a people laden with iniquity, children that are corrupters, the seed of evildoers. They have provoked the Lord.
It doesn't just mention evildoers, it mentions children that are the seed of evildoers - in this case, Violet's daughters.
Song of Solomon 6:4: Thou art beautiful, O my love, as Tirzah.
The inclusion of this verse could imply that even though Nell and May were "the seed of evildoers," their mother thought they were beautiful and loved them. In her mind, she believed she was protecting them. But still I wonder if part of her, on a subconscious level, still thought of them as "corrupters."
I'm pretty fascinated by the psychology of this woman. On one hand, she was loving and took care of Nell, but I'm not sure her motives were all that simple.
Granted, she seemed to be a good enough mother that Nell lovingly placed flowers on her dead body.