The issue is you’re not allowed to not include him. This is to prevent families from leaving disabled family members behind in their home country. There is of course a risk of your parents also being denied if he was included if he had high medical needs but the goal of PGP is family reunification. Ditching a disabled family members in a home country while the rest of the family moves/gets PR is not family reunification. What happens if your father dies or gets sick? What happens to your brother at that point? I assume your mother has been the primary caregiver of your brother for his whole life so his mother abandoning him to live in Canada seems pretty cruel.