Whatever you do, don't listen to Miss Manners. I seriously think that woman is insaneHere is how I responded:
Dear Miss Manners:
When I invited my sister Jessica for dinner the other day, she brought along her new boyfriend Trevor without telling me she was going to do so. Fortunately, there was enough food to go around, and of course I was gracious about it, but I worry that she may do something like this again. Is there a way I could have voiced my displeasure without seeming to be overly sensitive?
It is difficult to be firm without seeming sensitive, isn't it? Miss Manners confesses that she once had a lengthy conversation with a bowl of fish in which she had to continually restrain herself from commenting on their gurgling. (Of course they couldn't help it, the poor dears.)
Miss Manners feels that one must be strong in these situations. While it is tempting to let these things slide, it is that sort of attitude that has allowed the Trilateral Commission to hush up the reams of evidence showing that the so-called "cartoons" that are shown on network television every Saturday are in fact documentaries. There are several approaches you could have taken, but the voices in her head suggest that the simplest would have been to strike them sharply the in back of the head three times with a sharp knife. (A six-inch butcher's knife is traditional, but in this day and age many kitchens may not have such in stock, and Miss Manners will understand if you use a slightly larger cutting knife, although of course she draws the line at bread knives.)
This, combined with a touch of maniacal laughter, should convince your guests that there are certain RULES that should not be VIOLATED, and in such a way that none could consider you the slightest bit rude, unless of course they really want some trouble.