you need to provide some more information about your brother to get a better answer. the answer to these questions will help in answering your concern:-
1) when did you brother gets his PR approved?
2) at the time of PR approval what was your brother's marital status?
3) Did your brother mentioned about his first wife or that he is married in any of the official documents in canada e.g (tax documents, any immigration document, work permit, driving license, any visa application, health card etc etc)?
4) does your brother or her first wife has any document which translates that they both are married (e.g Pakistani id card, passport, visa application to any country approved or rejected, etc)
If canada has any information about your brother's first wife or that he is married then he cannot sponsor his 2nd wife until he prove that he no longer live with her first wife and is now officially divorced.
If by any means CIC finds out that your brother has given a false statement or he is previously been married they will not only reject his sponsorship application but will take a legal action and in result your brother may loose his PR status with a ban to enter in canada.
assume your brother successfully go through all the process by stating that it is his first marriage and in future his first wife mention his name in any of her document as her previous husband (it can be she herself coming to canada on visa, sponsorship etc) or she simply writes to Canadian gov, immigration about her issues or inform cic that her husband is getting married or that he was her husband... in any of such possibilities your brother will be in a Big trouble no matter even it happens after 20 years. misrepresentation and marriage frauds are considered as a serious crimes here in canada.
breakup of a relationship happens and it does not effect any future sponsorship application at all if everything is done legally.