My Canadian brother was convicted for fraud and sentenced by the US governemnt. There were 6 victims in all, 5 from U.S. and 1 from Canada, they were all involved in the sentencing hearing. He would like to return to Canada after serving his sentence. Can he be tried again for the same crime in Ontario?

Involved in the "sentencing" means nothing, what matters is was he charged with the crime, where did the crime happen, if he was convicted of the 5 in the US, but the one in Canada was never tired, but merely testified at sentencing, yes, he can be tried in Canada also.