The term that best describes the relationship between Mr. and Mrs. Fitzwilliam and Mr. and Mrs. Gardiner is "extended family." Mrs. Fitzwilliam is the sister of Mr. Gardiner and they share a family bond through their siblings' marriage.
The book "Mr. and Mrs. Smith" by Melanie Marchande falls into the romance genre. It focuses on the relationship between the two main characters, Mr. Smith and Mrs. Smith.
Mr and Mrs
Mr and Mrs is correct or Mrs and Mr is correct
Mr. and Mrs. (ex. "Hello, Mr. and Mrs. Evans" instead of "Hello, Mrs. and Mr. Evans")
Mr. & Mrs. Smith
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In the US it is normally Mr. and Mrs.
Dr. and Mrs. or Mr. and Dr.
I would address it to Mr and Mrs David Keys. For the salutation I would use Dear Mr and Mrs Keys.
Mr. used to be acceptable to address a household by just using the last name, e.g. "Mr. Jones". In modern times, it is typically to use "Mr. and Mrs. Jones" or "Mr. and Mrs. Timothy Jones".
Mr and Mrs its just more formal today in society.