I have no direct experience of this class of application but I have learned about it over the years by reading newsgroups and hearing from people.
The best place to start your investigations is the government page at Canadian Citizenship and Immigration - Family Class page.
Who is eligible for sponsorship in the family class?
You can sponsor a spouse, common-law or conjugal partner, or dependent children and dependent children of your dependent children.
Brothers or sisters, nephews or nieces, granddaughters or grandsons who are orphaned under 18 years of age and not married or in a common-law relationship
Another relative of any age or relationship if none of the above relatives could be sponsored, and you have no other relatives who are Canadian citizens, persons registered as Indians under the Indian Act or permanent residents and accompanying relatives of the above (for example, spouse, partner and dependent children).
Who is eligible to sponsor under the family class?
If you are a Canadian citizen or a permanent resident of Canada, you can sponsor your spouse, common-law partner, conjugal partner, dependent child (including adopted child) or other eligible relative (such as a parent or grandparent) to become a permanent resident.
Is every Family Class application approved?
No. They still have to pass criminal and other background checks, medicals, etc.