My wife got the comma-less form of the question in an email message the other day.

Problem:
Inclusion or exclusion of the comma affects the meaning of the question.

Explanation:
A comma in a sentence indicates a pause, so it can change the way that someone reads a sentence and therefore the meaning of the sentence.

My wife got the question as “How are you my sister?”, which effectively asked my wife how she could be the message writer’s sister.

However, given that the message writer is not my wife’s sister and knows that she is not my wife’s sister, what she should have written was “How are you, my sister?”.

This form with the comma would have

• let the message writer ask my wife how she was doing,
• used a comma to indicate a pause, and
• included “my sister” as a term of adoration or nearness.

Solution:
Include the comma to address someone as your sister and to ask her how she is doing. Exclude the comma to ask someone how she can be your sister.