What is the best route for my sister-in-law to move to the US?

A sibling's petition will not do any good, since it will take 12+ years to process. Sister in law can come in on a TN status if finds a willing and able employer and has the right university degree, as well as H-1B, J-1 or even O-1. She could also file for a Treaty (small) Investor visa: E-2 or even E-1, depending what she'd like to do.

The easiest way for a Canadian citizen to live and work in the US is either TN Visa or E-2 by owning and operating a small business.

I agree that a TN or E-2 visa would be the way to go, if she qualifies. She can, of course, visit you with just her Canadian passport but only for 90 days and she cannot work. A TN visa requires her to have at least a Bachelor's degree (the U.S. equivalent) and that she get a job as a professional. But it would depend on her job title and duties, and whether she has any prior negative immigration history or criminal history. I would suggest she consult with an experienced immigration attorney.

I would have your sister-in-law set up an appointment with a competent immigration lawyer so that she can talk to the lawyer in a confidential setting about the different options that might be open to her. It will take a good lawyer some time to evaluate all her options and decide what might work for her. I have run into Canadians who want to move to the US who turn out to be US citizens but did not know they were citizens; some Canadians can easily get green cards because of the Jay Treaty; others are eligible for NAFTA visas; and still others qualify for yet other immigrant and nonimmigrant visas. There's not enough information about your sister-in-law posted here for anyone to tell you what the "best route" would be. She needs a consultation with a good lawyer to determine that. US immigration law is extremely complicated.