There are few more categories of relatives other than immediate family members, who can be sponsored by a Canadian citizen or a Permanent resident. Only under certain terms and conditions you can be qualified as a sponsor.
You can think of sponsoring:

• Your brother, sister, nephew, niece, grandson or granddaughter, including the step-brother or step-sister ,who are orphaned, under the age of 18, and not married or in a common-law relationship
• Any another blood relative from father or mother side, if you do not have a spouse, common-law partner, or conjugal partner, or any one of the following living relatives below which you could sponsor instead:
  • • A child
    • A mother or father
    • Brother or sister
  • • Uncle or aunt ( family member of any diseased Uncle or Aunt is not covered)
    • Nephew or niece
• AND:
  you do not have any one of the relatives mentioned above who:
  • is a Canadian citizen, permanent resident, Indian (Native, registered under the Indian Act) OR
  • have an application for permanent residence that is being currently processed

You can sponsor only if you are:

• At least 18 years of age
• A Canadian permanent resident living in Canada or a Canadian citizen
• Meet the Low Income Cut-Off (LICO) which is normally required for last 12 months, where ever applicable
• Not in prison, bankrupt, under a removal order (if a permanent resident) or charged with a serious offence