The information you need is here:
http://www.cic.gc.ca/english/immigrate/sponsor/relatives-apply-who.asp
You can sponsor nephews and nieces if they are orphans and under 18 and not married. If that is not the case, then you can check if there is a possibility for you to sponsor them under the provincial nominee program where you live, see
http://www.cic.gc.ca/english/immigrate/provincial/apply-who.asp