My sister is Canadian and I'm might sponsor her to come to America. I know the timeline is lengthy - does her being Canadian help at all or does it make a difference? If she's undecided about coming, should I just go ahead and file, just in case she changes her mind in a year or two?

It will be a bit more than a year or two.

The only minus I can think of is that it might cause her additional srutiny on visits, but assuming she has strong ties to Canada that should not be a problem.