I've already had to explain a great-niece who's nearer to my age than she ought to be. A nephew who's older shouldn't be that much of a challenge.

[ of course, it's not how she's been explaining him at all. instead, she's called him her colleague's son. but perhaps a nephew, by blood or by choice, is an easier sell.

and if it's what he wants to be called? well! ]

We'll try it on for size. See how it settles.