That's not how the rules work. You can in fact sponsor your parents and they can fix the medical issue by having their TB treated.As I can't sponsor my living parents oversea in canada as you said because of the excessive medical condition of contagious (tuberculosis) and can be hazard to public of Canada so left is only my brother .if in other relative option I can sponsor one person regardless of age then why can't it be my brother ?when my parents are not sponsor-able because of this disease.
It sounds like they've chosen not to have their TB treated. That's their choice. It doesn't mean you can sponsor your brother.