We do not have specific information on how long you can drive a car registered and insured in the US can be kept and driven in Canada. We did find information at Transport Canada that states that under Section 7(1)(b) of the Motor Vehicle Safety Act, a motor vehicle may be admitted temporarily into Canada without complying with the Motor Vehicle Safety Regulations if it is to be used exclusively by a person entering Canada as a visitor or a person passing through Canada to another country.
Furthermore the vehicle cannot be sold or otherwise disposed of while in Canada and cannot remain in Canada for a period longer than the time constraints listed on the work permit, student visa or any other Customs documents. Once these time limits are exceeded, the vehicle no longer qualifies for temporary entry under section 7(1)(b) of the Act and must be exported.
As the Canada Customs and Revenue Agency also has requirements concerning the importation of vehicles, you may also try contacting them.
For specific information about the province you are in try contacting the Motor Vehicle Registry of Alberta. This government agency may be able to advise you on how long you can legally drive your brother's Lotus in Calgary.
For a Canadian car insurance quote follow this link.