Thank you for confirming.
You can only sponsor if your niece is orphaned. Here's an excerpt from the cic.gc.ca.
Option 1 – Orphaned close relatives
You can sponsor close relatives, related by blood or adoption, such as brothers, sisters, nephews, nieces, or grandchildren only if they meet all of the following conditions:
They are orphaned,
They are under 18, and
They do not have a spouse, common law partner, or conjugal partner.
Do you happen to know if some other province is inviting applications for this case?