Its about proof of relatives so, any reason would be trying to be over smart to VO. Its always better to say truth... This is my experience.... You can not give any stupid reason coz u dont make relatives in Canada overnight. They had given you enough time to submit your full docs and if u did the mistake so, its better that you write a nice cover letter with apology and VO will be surely convinced....
Do this before that start reviewing your application or you can wait for second AOR but I am afraid, if they'll put ur docs for review immediately after generating 2nd AOR