Can one family member's medical condition make the whole family's application inadmissible?

Yes. Because family members included on the same application, whether or not they are all travelling to Canada right away, are generally assessed together, an excessive demand finding against just one of them — often a child or another dependant — can make the entire application inadmissible, not only that person's own portion of it.

This surprises a lot of applicants, who assume that if the main applicant and most family members are healthy, a single dependant's condition simply gets handled separately or excluded from the file. That isn't how it works: because family members are linked on one application, a medical inadmissibility finding for any one of them is generally treated as affecting the application as a whole, unless that family member happens to fall within one of the narrow categories exempt from the excessive demand assessment.

Where a mitigation plan is possible, it typically has to address that specific family member's anticipated needs, but it's submitted and evaluated in the context of the whole application. Anyone with a family member who has a significant ongoing medical condition should raise it early and get the whole application's risk assessed together, rather than treating it as a side issue.