If your sister is sponsoring her visit including tickets, travel expenses and everything else, she needs to show a very strong bank statement and probably her financial well being in Canada. If mother is going to pay for her visit then she will need to show bank balances, savings etc and not just the minimum, it should be a healthy bank statement so that VO gets satisfied with financial situation. If someone else is paying (son), then he has to include all his details as well. As per your mother, anything on her name will be added bonus. Either kids back home, husband if married or savings, property or anything that shows that her life is well settled back home instead of planning to live in with daughter after getting in to Canada.