This is an extract of the email I received form AVO
Updated family information: you must advise us of any change in your marital status or family composition. All family members must be listed on your IMM8 application form. Failure to comply may result in a delay in the issuance of your visa.
Pls do I need to fill IMM8 if there is no change in my family size since I my initial application to CIC
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Fill the form & state in ur cover letter to them that there is no change in marital/family composition.