I just joined today and posted my story here, but parts of it can give some perspective on this if you have the time.
https://bpdfamily.com/message_board/index.php?topic=309805.0My wife has known for many years all the problems I've noticed, and when I finally found and researched BPD, it fit her to a T. At this point, however, she had already committed to go see a psychiatrist, and she knows I do my research, so I was able to "spin it" like the truth it is (sad, I know), and told her I care about her, and I was reading about this to better understand her and be there for her with better support.
I told her what I read was revolutionary in my understanding of her, and made me feel newfound compassion and empathy for her (all true, of course). I then asked if I could share it with her. She was triggered, yes, but reluctantly accepted. I read the mayo clinic's description (have since found better resources), and she was triggered by each symptom description on the list, immediately justifying each one, and attributing to something other than abandonment and sexual abuse, etc. from childhood. She kept saying those things didn't affect her.
But we got through the whole thing, with only a relatively minor episode, and it was worth it. It did take two sittings, because I jumped in on some of the explanations and tried to get her to see reason, which was a big trigger.
But if you can just get her to see a psychiatrist, you really don't have to say anything about BPD, if they are half decent, they'll see it very quickly. I didn't say a word at her psych appt, but I sat back a little bit and mouthed some things, or nodded or shook my head when my wife hesitated in answering. The amazing part was how my wife answered like she had a self-awareness I never knew existed, because when she talks to me (her trigger) all awareness goes out the window.
She really did well there, and I was impressed and proud of her. The doc diagnosed her with BPD with co-occurring depression, eating disorder, etc.
That is when things got much harder for her. That was two weeks ago, and she's been a 24/7 wreck ever since. The guilt of the problems her disorder has caused those she loves most in unbearable, and I console her for hours everyday. She bounces between denial and hopelessness and has a pretty big episode almost every day now. Yesterday was pretty good, though. She had her first DBT appt, and is already trying to practice whatever they told her, but it hasn't been smooth.
For the first time in 10 years, I have hope for the future.
So basically, having someone else whose impartial bring it up is better (if he's a main trigger) and if she doesn't want to see a shrink, maybe she'll see a DBT certified therapist?
Also, my step-dads ex-wife has BPD and is considered like the pariah of the family, so my wife experienced something similar to your SIL, in that she pushed back on accepting it for fear she's the "step-dad's ex" of the family now.