Just found this:
https://www.thewayimmigration.ca/immigration-tips/imm-5406-additional-family-info-guide
Leaving off step-children – your spouse’s children
are considered your step-children, and they must be listed on your IMM 5406 Additional Family Info form. Sometimes family situations are a bit complicated, such as when you married later in life and your spouse’s children were grown already when you married –
you may not be considered a step-parent for practical reasons in that situation. Our advice is to over declare, and still provide the information as if your spouse’s children are your step-children. IRCC can always disregard extra information if they deem that it’s not relevant, but if you fail to declare the information in the first place, you can run into problems with having your file returned as incomplete or in extreme situations, having to defend yourself against a misrepresentation allegation.