Canada tourist visa for younger sister -possible?

My wife wants to invite her mom and younger sister (19 years old, single) to visit us here in Canada. The dad passed away in 2014 so the mom is a widow and the sister is a dependent who lives with her mom. Her sister is a university student who will be on break from July until September.

I have been told that the chances of getting the visa approved for my sister-in-law is slim. If I provide documents showing excellent income (for myself only) will we be in a better position with the application? Is there anything else that we can provide to give assurance to the Immigration Officer that she will indeed be returning to her home country?