Don't mention that you have a sister in Canada, i guess this is what you can do. You can show that you'll come back by showing that your family is in Pakistan, your siblings studying or your parents having this xxxx business, show documents of property, business, your future employment in Pakistan after finishing your studies, anything you think might be relevant to show that you are connected and needed in your home country and your family needs you back, or your future plans in your country and how your course will be beneficial for you in your career in pakistan...
This is only my suggestions, may not be completely correct or relevant.