Sure you can write a letter but not showing their own funds weakens their application because why would they return. If they can’t afford to visit, you should be looking at a Supervisa if you meet LICO.What if the parents are both retired and have minimal savings, enough for their monthly expenses in India, but not to the scale of 10-15k CAD that a trip would require?
As a son, can i not write in my invitation letter that i will be purchasing flight tickets, travel insurance and taking care of their expenses here?