The letter, whatever you would want to call it, Letter of Intent, or Purpose or simply just a Letter is not a necessity but plays a crucial role in your application when your case might be considered fishy in terms of the documents you provide or if your situation needs to be explained to the visa officer so an interview or a straight-out rejection doesn't happen.
Please remember to NOT write the letter from the perspective of convincing the officer that you will return; especially when your mother and uncle are your guardians. That is simply because of the fact that they won't consider it your "strong" tie to your homeland, because how often does it happen that students from abroad would ever return to their country simply because their parents have property there? That provides a strong tie for your parents; but you might want to take my advice and downplay that part in your application and consider it one of the "weaker" ties. Also, please don't just FLAT OUT say you will return; because would beg questioning into the matter of whether you won't return.
Explain your situation thoroughly but focus more on your goals in life and how they correspond with your country; all the while also convincing the officer of the benefits of Canadian education to your work and job prospects; these are obviously prospects within Pakistan. Also consider the fact that your father is not anymore; how must life be affected when you're gone and how you want to do this for the betterment of your field of work; education and gaining international experience to cope with changing trends should those ever occur.
I can't stress out enough the fact that one needs to write a very convincing and personal letter. I think some cases are simply accepted on the genuineness of a letter even if funds or other information seems rejective (though one must always make sure that doesn't happen)