Hello Gymrat: I see this is your first post here on PC. So... welcome to PsychCentral! I hope you find the time you spend here to be of benefit.

I'm sorry you & your husband are faced with this most difficult dilemma. Sadly, while I'm certainly no expert with regard to this sort of thing, I personally doubt there are any simple ways to make your sister-in-law see the truth. She's an adult. And so long as she's not an imminent danger to herself or others, she pretty-much has the right to do as she chooses. The only remedies that would potentially be available, I would guess, would be legal; if in fact there are any.

Having written that, here are links to some articles, from PsychCentral's archives, that may be of some interest on the subject of helping someone who does not recognize they need help:

https://psychcentral.com/blog/11-way...one-in-denial/

https://psychcentral.com/lib/how-to-...sed-loved-one/

https://psychcentral.com/blog/how-to...essional-help/

Also... here's a link to an "Ask the Therapist" column wherein the writer was asking how to help a daughter. I include this column because, in her reply, the therapist recommends a book titled: I'm Not Sick, I Don't Need Help. The therapist writes that she considers it a must read:

https://psychcentral.com/ask-the-the...iple-problems/

Here's a link to a review of the book on the Friends for Mental Health website:

A review of "I'm not sick, I don't need help!" - Friends for Mental Health

I don't know, of course, if you're here simply seeking advice with regard to this particular concern or if you plan to continue posting. However, should you be planning to continue on (we hope you do)... may I suggest you introduce yourself over on PC's New Member Introductions forum? Here's a link:

https://forums.psychcentral.com/new-...introductions/

There's a lot of support that can be available here on PC. The more you post, & reply to other members' posts, the more a part of the community you will become. Plus there are the chat rooms where you'll be able to interact with other PC members in real time (once your first 5 posts have been reviewed & approved.) So please keep posting!