Can a family member become inadmissible to Canada because of another family member's criminal record?

Yes, in a specific and often misunderstood way. Immigration law treats certain family members as part of the same application even if they aren't personally applying to come to Canada right away — commonly a spouse or dependent child who isn't accompanying the main applicant. If one of those family members would themselves be inadmissible, that can make the whole application inadmissible, not just that individual family member's own future application.

This is different from a family member simply having a difficult relative; it specifically concerns people who count as your family members for immigration purposes under the application, whether or not they are travelling with you. A parent's serious criminal history, for example, can affect a child's or spouse's case if that parent is included as a non-accompanying family member on the same file, even though the parent isn't the one seeking entry.

Because this rule catches people off guard — many assume only their own record matters — anyone with a family member who has a significant criminal history should flag that person and get advice on how it affects the application before filing, rather than leaving them off and hoping the issue doesn't surface.