Quoted from Cic's website:
Who you can sponsor
You can sponsor:
brothers, sisters, nephews, nieces, or grandchildren who are orphaned, are under 18 and do not have a spouse or common-law partner,
relatives of the above (for example, spouse, partner and dependent children) who will come with them to Canada, or
You can sponsor one relative of any age or relationship, but only if you do not have one of these living relatives who could be sponsored as a member of the family class:
spouse or common-law partner,
conjugal partner,
son or daughter,
parent,
grandparent,
sibling, or
uncle, aunt, nephew or niece, and
you do not have any relative who is a:
Canadian citizen,
permanent resident or
registered Indian under the Indian Act.
Who you cannot sponsor
You cannot sponsor other relatives, such as brothers and sisters over 18, or adult children who support themselves financially. If they apply to immigrate under the Skilled Worker Class, they may get extra points for having a relative in Canada.