@brownboy1
I would recommend that your brother-in-law, sister and niece should get visas for Australia first + visit Australia first together as a family.
IRCC would question why they want to visit Canada for 10 days (considerable expense, traveling across the globe, all three family members applying, no family/relatives in Canada, no previous travel, etc).
Also, you offering to pay for their expenses would make their financial situation to seem weak. It would be best if your brother-in-law proves that he can afford the detailed day-by-day itinerary + plane tickets (for all three) + hotel + expenses.... with his own finances. Your bank statements/passport copy/provisional flight bookings can be included with a statement that says you will be sharing the expenses as-and-wherever-required.
For your brother-in-law:
Employment: Employment letter + NOC/leave approval letter stating specific dates + 6 months' salary slips + 2 years ITRs + work commitments to return to with evidence (projects scheduled after he returns + evidence that no one else can do the work he does) + PPF statement + appraisal/ promotions and salary increment after he returns + benefits due to his employment + 6 months' bank statements + any financial investments (FDs, RDs, mutual funds, shares, EPF, etc) + credit cards and 2-3 months' credit card statements (not debit cards)
Detailed day-by-day itinerary as tourists with expense estimates (thorough research for sightseeing, cost of meals/ local travel/ travel insurance/ expenses/ surplus funds).
Proofs that he, his mom, his sister live at the same address + evidence that he pays for the utility bills/ rent (?) or property taxes/ medical insurance for his mom, etc.
Birth certificate of the niece + copy of your sister's passport + provisional hotel and flight bookings (not confirmed bookings)
For your sister:
One complete set of financial documents of your brother-in-law + a letter of financial support from him + any joint bank account statements and investments + copy of your brother-in-law's passport + birth certificate of the niece + detailed itinerary + provisional hotel and flight bookings (not confirmed bookings)
For your niece:
I would recommend they do not apply for your niece. They can apply for the niece after they are granted TRVs