Bad Manners and Brimstone

My point was that "plus-one" quite often gets used to refer to what one might call secondary guests: the people you aren't close enough to to invite in their own right, but are being invited because they are married or otherwise attached to people who you are close to.

As in: suppose I were getting married and was inviting all my first cousins along with their spouses, live-in partners, steady boy/girlfriends - some of whom I know very well, some only a bit, and others who I only know by name. I'd put these people's names on the invitations - I'm old-school enough never to send invitations to "you and a guest" - but when discussing numbers, working out the table plans, and stuff like that, I'd say things like "We could put my cousins and their plus-ones on table 4 along with yours' and just using that phrase certainly wouldn't imply that if my cousin Sukey's husband couldn't come she was at liberty to decide to bring someone else!

So we don't know, and from the published letter it's very possible the aggrieved writer didn't know either, how the invitation was worded and whether his SIL had the right to choose a substitute guest for herself or not. So that whole line of discussion is a red herring.