The New Yorker: "Twins" by C.E. Morgan

Whenever I discover that a New Yorker "story" is an excerpt from a book, I lose my enthusiasm for commenting on it, no matter how well written it is. It's not that there's anything inherently wrong with excerpts, but they should be approached and evaluated differently, which is why The New Yorker should disclose that it's an excerpt.
So here we have an excerpt, which we know only because C.E. Morgan tells us so in the Q&A (link below). And of course you only see the Q&A if you go online...