That's the complexity of the case. A Canadian does not need to prove why they need to be in Canada. And yes, the family can support themselves while in Canada.
Another thought, anyone been sponsored usually sign that they would do their best to be productive....that can be an undertaking of some sort.
I guess they would never know how far they can go except they try. A Canadian citizen is involved here.
The person being sponsored cannot sign an undertaking. That is not how the regulations and laws are written.
The privilege to sponsor a family member attaches to the Canadian. If the Canadian is not legally able to undertake to support the family member, they cannot sponsor them.
Additionally, the Canadian must satisfy IRCC that they have the ability to
meet the undertaking - that is, pay for the expenses of living for the sponsored family members for the 3+ years of the undertaking.
Are you an agent or consultant advising these people?