The U.S. company is the petitioner and pays the government fees on the USCIS schedule. Allocation of professional, relocation and salary costs between the affiliates is an accounting matter that should match what the petition says about who employs and pays him.
Let the accountants allocate, but keep the story consistent
Hypothetical example: the siblings intend the Canadian retailer to keep paying the general manager and to invoice the U.S. company for his time, because the Canadian payroll is simpler. That may be workable, but the petition must describe the employment relationship truthfully: who directs his work, who pays him, and under which entity's benefits he is covered. An arrangement where the Canadian company pays but the U.S. company controls needs to be explained rather than glossed. Beyond salary, budget the petition and any premium-processing fee on the USCIS schedule, legal fees, relocation and housing, vehicle and driver licensing in the state, health coverage for the family, and the cross-border tax cost of a mid-year move for both the transferee and the companies. Agree in writing which company bears each item and what happens if the petition is refused or he returns early. That agreement is useful evidence as well as a commercial safeguard.
Hypothetical example: a Canadian marine-equipment wholesaler transfers its regional director but leaves the relocation budget unsigned until after the move. The assignment letter, payroll records, and intercompany agreement should identify which entity bears salary, tax equalization, insurance, and a return trip if the role ends early. That review also confirms the transferee completed one qualifying year abroad in the three years before filing or admission. L-1A status has a seven-year maximum, making a long housing lease or school commitment a business decision rather than a routine expense.