For TRVs, your parents must demonstrate that they can afford the visit on their own. In their situation, it would have helped if your brother could support their visit, but as you've mentioned this is not possible.
1. Does your dad get a pension every month? Do your parents have any financial investments (FDs, RDs, shares, mutual funds, LIC, etc)?
No pension, only some FD that too amounting less than 10000 CAD, that too split between both parents, not solely in one person name.
They also own a home, but in name of mother again for tax reasons.
2. Do post the list of documents which you intend to submit
From my side, I can show my T4, salary slips, job offer letter, bank statements.
My advice:
1. Even for a TRV, your dad must be in reasonably good health + covered by an overseas travel medical insurance policy. You wouldn't want to run up a huge bill if he unfortunately has to be in hospital.
He has been ill for few days more than 3 years ago, but now he seems stable. Did not have any further complications after that and has actually improved personally. Only his test results show what I already described, and that doctors said is going to continue.
I will make sure I have his insurance covered with pre-existing coverage before he comes , actually even before I apply for him, to. make his case strong .
Don't risk applying for your dad or his visit to Canada if his health is not too good.
2. Hope you're keeping your parents' visit to just 3-4 weeks max. and not 6 months on a TRV followed by an extension.
Don't intend for extension for now, but want them to live with me for some time. I want to make them happy and give them what I could never do I my whole life yet.
3. Alternatively, apply for a TRV for only your mom.... wait until your dad's health improves and then apply later for a TRV