DEAR ABBY: Some friends of mine and my husband's invited us over for a sit-down dinner at their home. The friends are acquainted with my husband's parents and extended a written invitation to them as well, which I thought was very nice.
Because the friends live some distance away, we planned to carpool there with my in-laws. When my in-laws arrived at our door, they had my sister-in-law with them. (She's middle-aged and still lives at home.) I was mortified because I knew "Sis" hadn't been invited.
I promptly called the hosts to give them a heads-up and an apology, and offered to bring more food. My mother-in-law overheard me on the phone and took great offense and said they weren't going. She said my husband and I were rude because we treated Sis like a criminal. She asked repeatedly before storming off, "What's one more person gonna hurt?" Who was in the wrong here? -- BAFFLED BY LACK OF ETIQUETTE
DEAR BAFFLED: Your mother-in-law was wrong to bring an uninvited guest with her. Assuming you quietly telephoned to warn the hosts, and did not embarrass your sister-in-law by doing it in front of her, your mother-in-law was wrong again in taking offense because you tried to keep the hosts from being caught flat-footed. It was extremely rude to punish the hosts by leaving them with empty seats at their table at the last minute.