Who’s getting married – you or your brother?
Also: Chiropractic “works” for what exactly? If your future SIL is claiming anything other than back pain, let the burden to provide “evidence” fall upon her. Don’t forget to insist that she accompany any contributions with peer-reviewed studies.
Going back to the OP, I ask why you are so determined to change her mind about this? How does her belief affect you?
Are you trying to make her mad before she is part of the family because you do not like her?
Are you going to do this with any other thing you think she sees the wrong way in the world?
It seems to be the consensus in this thread that it can help back pain. So not totally woo.
Just how important is convincing her you are correct? Willing to destroy your son’s marriage plans?
I have done things in an airplane that can not be done according to the experts. I have learned to not argue about, or even hint that I disagree with the experts.
It seems there will always be people who will go to astrologers to get life decisions made for them.
Anyone ever change an adult’s mind who believes in it to change their mind?
Ignorance & stupidity, two different things, can never be totally eradicated.
If the future SIL will argue about it from the position of a believer, your arguing about it will not end well. IMO
YMMV 
Gee wizz A son. I never knew I had one of those. And he is going to marry someone who will be my sister in law. So that must mean he is my son and my brother -hark, I hear banjo music.
Oh and I have issues with her blatant racism but I guess that that sort of stuff should never be challengef
You’re picking only that one error out of all that he said? Really?
Guy makes some solid points and you’re pissing all over a mistake like that. Look the point is this - you can’t convince people like that, and if she’s a racist also, why the hell do you even want to? Feeling like you “won” over a future family member who from the sound of it is a worthless piece o’ crap is a futile endeavor.
Chiro “works” in that the people who are happy with it, got what they wanted from it. The people who are unhappy with it, didn’t. Anyone who claims it can do anything other than provide musculoskeletal warm-fuzzies (potential injuries notwithstanding) is deluding themselves. End of story.
I knew a guy who’d been suffering from worsening back pain for some time. He kept visiting the chiropractor & it only got worse. (I saw him at the time & he looked shockingly bad; not being an intimate friend, I didn’t intervene.)
He gave up & went to an acupuncturist. (Yeah, he was an old hippie.) Who gave him a quick exam & told him he needed to see a real doctor–immediately. The problem was pancreatic cancer. Even earlier diagnosis might not have helped, but at least the MD’s gave him access to the really good drugs for what remained of his life.
In this situation, the acupuncturist was not the quack.
*Never *use “alternate” medicine initially for serious conditions. Even quite orthodox MD’s occasionally give the OK for what they call integrative medicine–to supplement standard treatment.
My Dad could say with complete honesty that his sister in law’s son in law was his brother in law.
No death, divorce, banjo music, May/December joinings or illegal or amoral acts involved. he he he