The denoument, which I skipped to out of boredom, is that the strange behaviour of one of the teenagers is due to him believing he is possessed by a succubus due to constant masturbation due to a tight foreskin.
"You see, Mrs Stern?" said Ptolemaous. "Your Hellenophilia nearly cost Marius his sanity. You didn't trouble to notice that his uncut little foreskin was too tight. The result was, when he began to get bigger, compulsive masturbation, more and more compulsive, to relieve discomfort; and the result of that was the construction of a diabolical fantasy by which he sought at once to symbolise his situation and to shift the blame from himself." [...] And to Marius. "Have you understood all this?"
Marius nodded. He went to his mother, kissed her gravely on the eyes and linked his left hand little finger with her right hand one.
"Darling Mummy," he said. "Please may I be circumcised?"
"I have no doubt," said Isobel superbly, "that your father will know how to arrange it."
Now, what I think is interesting about this, is that it is the clearest example ever of the brain-eater attacking a non-SF writer.