The odd thing is that this is still coming up in 2017, with that sort of fake-helpful phrasing from the letter writer. I know Miss Manners has addressed it before, including noting that even in the days when more people were happy to be addressed as "Mr. and Mrs. John Doe," the multiple-line form was appropriate for households other than married couples, including such utterly respectable situations as a widow living with her daughter. They might have shared a surname, but that sort of old-fashioned style didn't call for "Mrs. Rock and Miss Ruby Star."