Since she denies that anything is out of the ordinary, how can I best offer support? My goal is for her to eventually seek therapy, though I know DBT is expensive, or to grow to a level of self-awareness where she can admit her true feelings and possibly practice mindfulness, even if on her own.
I believe my SIL is N/ or BPD. I knew something was off with her pretty much right off, but it took me awhile before I found information about Narcissistic and Borderline Personality Disorders. Ironically I found out about NPD while looking up how to deal with bullying from her oldest son... .which led to Stop Walking on Eggshells, and it was a huge lightbulb moment. Yes! That's what we're doing: walking on eggshells around her. I had never met anyone as controlling, emotionally volatile, and so prone to wild confabulations when confronted.
I've talked to my SIL about BPD. And I've talked to her about taking responsibility for her feelings and for developing coping skills. She hasn't been very receptive. In all the years of being on this website, it's pretty rare for a person with BPD to get therapy and improve. It's not a bad thing to "hope" that your sister will some day get therapy. But I don't think you should make it part of your goal. The only goals we can have are for ourselves --learning how to best respond to a person with a personality disorder, and defining and strengthening our boundaries, our needs and our personality.
I wanted to add, Stop Walking on Eggshells talks about a reflective way that we can talk to a person with a personality disorder that can prevent conflict from escalating. I have to admit, when I first read the book I was just so fascinated to read that there were other people like this, other people who had the same emotional response of walking on eggshells, that I didn't really absorb the advice it gave. By chance another sibling started to do this reflective listening with her. And it's something that we occasionally use. Even thought she has tried to use reflective listening/empathic listening as a means to manipulate and get her way, it's really been the most helpful tool. So I strongly recommend reading up on it or looking up videos on how to use this method. When we repeat back what she says, it calms her and she's able to better listen to us and she's more likely to acknowledge our needs. From there, you'll be more likely to work toward a conversation about seeking therapy.