Canadian Marrying American

My sister is Canadian. Her Fiancee is American. She has been living and working in the U.S. For almost 2 years with what I think is called and I-94 work permit, which she renews annually. They just got engaged 2 weeks ago, and are hoping to be able to be married by the end of the year.

Does anyone know if she will have to stay in the U.S. After the official wedding, or whether she can return to Canada. I`ve been told you have to stay there for 12 months, but because she is already living there, would there be a different process, legally?
Has anyone had any experience with this situation? Any recommendations for websites to look at?