Are you using 2 accounts? I just answered a similar question this week. Yes certain school boards are sanctuary school boards but that still means your sister would not be legally able to attend school in Canada without a study permit. The children living in Canada without status and attending school usually have their whole family in Canada, can’t leave Canada, their extended family will face difficulties entering Canada to visIt, work or study. If you niece attending school in Canada without a study permit she’ll either have to try and renew her visitor status every 6 months. Given that she should be attending school in her home country and loving with her own family it will be difficult to get extension to remain in Canada. if she chose to remain she would be remaining in Canada illegally and breaking immigration laws. This will have longterm impact on her future and likely your future. You will struggle to get any of your common relatives approved to visit, study, work, etc. for a very long time. What will happen when she gets to college or university? She would need a study permit there are no sanctuary colleges or universities. She would also be an international student.
what you are suggestions is a very bad idea and comes with serious negative implications for her future. If she wants to study in Canada and if you have the funds she should consider coming at the postsecondary level.