Windy, there really is no "winning" in an argument with someone who is "always right." A helpful quote to remember is “I never make the mistake of arguing with people for whose opinions I have no respect.” Edw. Gibbons.
Also, on a more wicked note, refusing to engage your sister-in-law will drive her batty. So perhaps you should simply smile sweetly, and tell her that you both obviously have your own opinions so there is nothing further to discuss on the topic. Then just walk away, or divert the conversation - and reapply repeatedly, always calmly and quietly - either she'll be smart enough to finally shut up, or only one of you will look sane in that interaction.