@starzibal I recently got a visa for a couple and their son from India. The couple's daughter was pregnant and in toronto. Similar case to your niece. They had showed bank statements of couple (both husband and wife), total of around 38000 CAD for 3 months visit for 3 people. You can use this to calculate that your niece's mother will need 38000/3 = 12000+ cad for 3 months.
I am not saying that this is the minimum funds or not. They had good funds available in bank statement so they were able to show it. if the woman has less funds, can she and husband apply together if husband has good funds? check this option.