Hi
before applying u have to either adopt ur niece/nephew and the only way u can sponsor them is if u dont have anyone landed relative here eg. husband parents sisters/brothers niece or hephew who can be sponsor as a member of the family class u also have to meet the lico and a steady income.
Sorry you are wrong, you can sponsor nieces and nephews under the age of 18 who are orphaned. The application can be really quick or it can be slow if there is no evidence of the death of the parents. Stating that one can't be found will really slow the application down. As they are under 18 there is no security check/criminal records check which helps in the processing.