@Linear M
Do include a detailed day-by-day visit itinerary with expense estimates.
1. Property: Property ownership documents + property tax receipts + valuation
2. Any financial investments?
3. Do your sisters live in the same city as your parents? Grandparents?
4. As your sisters are married, they are not family ties to return to from a visa perspective. Nonetheless, do include a 'Family ties to Pakistan' explanation with sub-sections for 'Immediate family' mentioning your sisters, spouses and their children (Name, address, DOB, nationality, relationship).... and other section for 'Extended family' mentioning your parents' siblings, cousins, etc (no details are required). This would prove they have more family in Pakistan than in Canada.
5. Do your parents have any compelling reasons to return to Pakistan? Any family events?
Don't go by the refusal reasons stated.
Your mom can include a Travel History explanation: Visa details + entry/exit dates + purpose of each visit + color scan the US visa and US entry stamp.
Is her US visa still valid? If yes, does she intend to visit the US as well?
Also:
1. Copy of your passport
2. Copy of your marriage certificate
If you meet LICO, you may want to consider applying for their super visas.
But applying now implies they don't mind visiting in the brutal winter, right? Maybe they should apply a few months later for a summer visit so that this point will seem convincing
1. Who owns the property?
2. Does your mom have any finances of her own?
3. Compelling reasons to return to Pakistan?
4. IMO, your mom possibly has a stronger application because of her US visa/US visits. You may want to consider applying for your dad first.... and after he is approved then apply for your mom
5. Do prepare individual cover letters and reference evidence