PGP before your brother turns 22 would be the way the way to sponsor your mother and brother. If your brother is able to secure a study permit he would be able to try and qualify for PR on his own and then you could attempt to secure PR for your mother through aw PGP application. Whether normal TRV or supervisa your brother may run into issues when applying for his study permit if his family ties are considered stronger to Canada. Your mother may also run into problems obtaining a TRV if both her children are in Canada. Showing that she will return home after her visit will be more difficult. Would suggest always having a plan B and not making concrete plans until you have secured a visa or permit.
