Cullen's mom initially seems like the strongest member of the family in the face of tragedy. When Oslo dies, she goes over to take care of her sister every single day to make sure that she has company and food, and when Gabriel goes missing, she doesn't fall apart like many moms would. Instead, she acts as if everything is normal and like Gabriel will walk through the door any day now. She even continues to go to work and cut hair for people. You might say that Mrs. Witter keeps the faith.
For a while, anyway. After a couple months, she starts to crumble. When Gabriel's disappearance finally sinks in for her, she is incapacitated by it; she can't go to work, eat, or shower. She just sleeps in his bedroom and mourns his loss. Her faith, it seems, hits its limit.