The Tension of a Nun

In the end, it is Sandy going to the headmistress and explaining that Miss Brody was the instigator of the death of one of the girls here. She doesn't seem to have any particular regrets about it. I don't think it's told to us in the novel why she joins the nunnery. Do we know why she becomes a nun? I don't recall it ever being mentioned. It's just sort of something that happens.