I couldn't have put that talk to her parents any better myself. You hit on all the key points.
If they would be willing to sit in on a MC session that would even be better.
I really think if you booked a MC session with someone MARRIAGE FRIENDLY in your area who DOES READ Davis, Glass, etc and is on the same page as you then you could invite her parents along to hear as well.
My bet is the MC would love to be able to talk to her parents as well to give them some advice on how to bring your wife around. Her parents mean well, but they have no education in marriage science, and you just started reading recently yourself.
I am wondering how your parents feel about their daughter, your wife's sister, telling you to give up. It sounds like the sister is not on the same page as her parents...her parents could try to talk some sense into her sister maybe, I dunno.
But this MC session would have to be private, you can't tell anyone about it. If your wife found out you went to counselling with her parents she likely would be quite upset.
This is just an idea, I will see what the jury here have to say.
So :
1. Well done with the parents pitch. 2. Find out if her parents can turn her sister around. 3. See if you can get a MC session booked. 4. Go to MC yourself 5. Ask if bringing her parents in to hear her advice would help.
My bet is a MC can give you and her parents better advice than we can. But go first YOURSELF to gauge where this woman stands on the matter. There are MANY BAD THERAPISTS out there.
My thinking is that her parents while hearing your well put pitch might be moved, it might not hit the mark. I think them hearing from a MARRIAGE THERAPIST might have more punch, even if the MC only confirms what you have already said. I think if a professional MC said that them letting their daughter make her own mistakes is a bad idea they may add more pressure on her.
ALTERNATIVELY, if your parents are church going people a religious authority again on the same page as you might add some punch. Some people listen to professional advice, some prefer someone with a fancy black suit and collar. It matters little who is is. What you want is a) someone they respect and trust and b) is on the same page as you (ie save the marriage at all costs)
You did good son lol
However, the professional authority may have more impact, simply because of who they are.
ALSO, if her parents SEE YOU going to MC and go there with you, this just really puts a solid stamp of approval on you for them. I would think any parents who went TO a MC SESSION with their son in law would just beam. You are maturing a lot, this is a chance to give her parents a front row seat.