Your older daughter sounds a lot like our younger.
Our younger daughter is 29 now, has a good job, a few good friends, and I think is past the worst of it. She was a cutter through her teens, she left home for a couple of years in high school to live with her biological father (who had no custodial rights, but that’s a story for another day), and she is ever holding herself to nigh-impossible standards. She has struggled for a long time with various eating disorders and body dysmorphic issues. As far as I know, she has never been diagnosed with BPD, but she has had more than her share of treatments for various depression- and anxiety-related issues. Also, as far as I know, she has not had any suicidal ideation, but in all honesty it wouldn’t surprise me if she did, and we just didn’t know about it.
And I wish I had some kind of magic bullet or sage advice to give you, but I just don’t. I still worry about our daughter, just not quite to the degree I used to. She’s getting better at channeling her obsessive tendencies toward more…productive?..pursuits. She was an ultra-runner for a while, she is very career-focused, and insanely careful about her finances (and obsessed with her credit score; watching her struggle with wanting to increase her credit score to the max while simultaneously not wanting to hold any debt at all was actually kind of funny). I know she still has her dark times. She doesn’t talk to me about them as much as she used to, but she does periodically, which I take as a good sign.
The only thing that seemed to occasionally work with our daughter in terms of guidance/advice was to gently steer conversations such that she would, herself, arrive at a new insight, or admit that something was a problem. She’s whip-smart (like your daughter), and can smell a lecture or too-strong advice coming from a mile away. Being too forceful or confrontational is a sure way to shut down productive conversation. I spend a lot of time just listening, and when I do speak, it’s often to ask an open-ended question that will draw out more from her, or to say something about how I feel, or what I do in a certain situation, while making it crystal clear that I’m not saying that she should do a certain thing or behave in a certain way.
There are no certainties in this world, and I can’t predict what will happen with your family. I can say that, for myself, there were a couple of years where I wondered if I would ever see my daughter alive again. Today, I actually consider her a friend, and I look forward to her visiting. I wish the same for you and your relationship with your daughter.
For the record, we worry about her older sister, too, but for entirely different reasons. That’s for another time.