It's OK to use even if you have only one wife; "dearest" here just means "very dear". Similarly, you can say "My dearest Mary", "My dearest sweetheart", and so forth, even though there is only one candidate. " wording feels rather formal and literary.
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tydexwang
Thank you for your reply, Mr. Wordy. How about "My best wife" and "My sweetest wife"? Do they sound OK?