Yes! Maintaining your boundaries is an act of love for yourself. Letting them down would accomplish nothing for your sister or your family but just hurt you. Family dynamics are so odd. My parents knew there was something wrong with my sister’s behavior but also shrank away from seeing it for what it was. And my mom wanted me to play that savior role that is actually impossible to play.

Hi Foolingmyself
There is one sentences you wrote that I think is very important:

She is not competent and has poor executive functioning. 

This is also the case for my DD. It makes all the difference in the world. Some young people with BPD are quite competent - therefore it is quite legitimate to set a boundary re getting a job, doing some study, attending appointments etc.

When there is poor executive functioning, the impulsivity of BPD is not restrained by reason - so the person is very much at the whim of their impulsivity. Similarly the need to avoid intense abandonment is so very strong - so when the opportunity to attach to a partner presents itself, there is no ability to balance that in the light of others' needs eg the needs of the child.

This is a huge factor in how to manage BPD. Things that will operate well for the executive functioning BPD, will not work for those with poor functioning.

You really know your DD very well and have clearly been able to observe and identify not only the BPD but these other factors as well.

When my DD was about 15 a paediatric psychiatrist spent a short time with her and identified that she would probably not be able to hold down a regular job etc. Things that proved to be true although for a long time I tried to help her to be able to do these 'normal' things.

It is good that you can sleep and that you are fairly peaceful in these circumstances. What the future holds is perhaps predictable - perhaps not. Whatever the case, this is the moment to breathe and appreciate your own life. Yes you can also be very proud of the effort you have put in to support and love your DD

What you're going through is so common here- everything feels like it's almost perfect, yet there's something missing.  And that's the small voice in the back of our brains reminding us of the good times, which might have been 10-20% of the entire relationship. 

All I'd say is, try to remember the other 80-90% as well.  The good times were good, but the bad times were equally bad and that took up most of the relationship.  It never balanced.

One thing that has helped me is the chant or affirmation that reality is more beautiful than memory or fiction. Exiting the fantasy relationship is hard though. Intermittent reinforcement and the high of cyclical crisis/resolution does something to your body. I have felt very detached and flat at times. I hope things get better for you and by the sounds of it/ the progress you are making, they will.

It would be great if our kids marriages were restored and were healthy.  My son and his ex do better separated. And still there is conflict.  They have tried couples counseling, individual counseling for short periods.  His ex has been living with another man for several years. My grandkids have places to be with each parent with less drama. When my granddaughter was little she begged to live with us. Sad but true.

I mean, is there any chance of him saving the marriage?  That's always the best case scenario but it start with him getting humble and winning her back.  Maybe it even leads to taking therapy seriously.

My BPD daughter is 27 and she's technically still married to a guy she rushed into at 20.  They made it two years and she's dated women ever since.  Part of me hopes that they could reconcile someday.

 That has been my experience with my son when his marriage of 15yrs ended. He was suicidal and was hospitalized for 2 weeks.
His relationship with this girlfriend has been almost 4yrs. Breakups are hard on anyone but someone with the fear of abandonment and feeling terror over being alone it is crushing. My swbpd will text I love you Mom - quite often. Though on one hand that's wonderful on the other I feel he needs the confirmation he isn't alone and that he's loved. He and his younger brother share a house. His 14yr old son stays most every weekend. His 20 yr old daughter is working and going to school but does stay in touch occasionally. I'm glad for all those people in his life. I feel like I'm his emotional security blanket in many ways though. I'm there for him but want to also not take on his neediness. His ex wife who is like a daughter to me, told me he has called her crying when he's in a really bad place. As of now - I'm kind of in the dark in how to navigate this.

I think that because this is not my first time having her split and then physically split, I’m just not as shocked or shook as a I normally will be. I feel relieved and I don’t seem to be ruminating over it. I think that over the years I made a very strong effort to improve my own self care and build my own identity outside of being her mother. The first time she did this I was a mess for six months straight. I didn’t know who I was outside of being her mom and I felt very lost. I’m not going to say I wasn’t shaken but the borderline behavior resurfacing after about a year or so of relative peace and quiet.

Something happened and I’m not sure what to trigger this. I happened to check my Progressive snapshot report for my car which she was using and I saw that she was speeding the same night the initial problem occurred. Something got switched in her brain and it was only a matter of days before she went full split and demonized me. I’m feeling very peaceful but occasionally I think of my beautiful grandbaby and I miss her sweet little smile. I have her blocked because I just don’t want to deal with any back and forth. I feel like she will just be blaming me and trying to engage me in an argument to bolster her lies. I’m staying low key. I haven’t cried, I’m sleeping okay. I’m honestly too calm about it all but that must just be from the fatigue of dealing with being on the receiving end of her borderline personality.

It definitely weighs on all of us, especially while trying to find that balance.  I deal with it a lot better these days but there will always be challenges.  In time, you'll find the right distance to keep and how to "balance out" the relationship where you're involved in your son's life but not at the center of it.

You mentioned a break-up, those were always the toughest times for my family and it often led to an in-house stay for my daughter.  In fact, her last breakup was the first time ever she didn't need to be hospitalized.

OK, excluding the the baby and abusive childhood, I could have written the exact same thing about my adult BPD stepdaughter, when she wasn't getting treatment.  Correction:  when she was skipping her psychiatric appointments but we were billed for them anyway.

My observation is that weed consumption turned her my stepdaughter's already questionable executive functioning into total dysfunction.  When she was using daily, it seemed she couldn't think straight, let alone plan, let alone do simple things such as remember to pack a bag for a weekend trip.  She had bouts of paranoia when under stress.  I think weed turned her occasional procrastination into total avoidance of life, and completely sapped her already low motivation.  Between weed and TV, she made herself comfortable retreating from the world and any adult responsibilities.  She was able to do this because she was living in my house for free, with no contributions or responsibilities, either.  Her dad gave her money (out of guilt) so she wouldn't go without.  Yet without the necessity of working for money (not even a simple household chore to earn an allowance), she basically locked herself in her room and stayed entertained with weed and screens.  At the same time, she was seething mad and took out her frustrations on anyone who would listen.  Since friends wouldn't put up with her hostile behavior, she lost every last one of them.  She became estranged from her entire family--siblings, her mom, her aunts and uncles, cousins, grandma, etc.  She'd only talk to her dad and me because she needed free housing, money and other support.  There was no love or concern there, it felt like a purely transactional relationship.

I felt that my stepdaughter was compelled to say the nastiest things about her parents to convince herself that she was victimized and traumatized.  That way, she could embrace the identity of victim and escape the deep, looming shame she felt for sabotaging her own life.  She is the one who made bad decision after bad decision:  quitting school multiple times, quitting work multiple times, getting herself evicted multiple times, getting herself in dangerous situations, losing all her friends, quitting therapy, committing various acts of self-sabotage.  She blames her family, even though they had absolutely nothing to do with any of that.  Deep down, what she was doing was trying to shift the blame for her own dysfunction to family.  The barrage of accusations seemed cruel, constant and yet also delusional.  What I saw was that the barrage was mostly a deflection, a distraction, a sure sign that my stepdaughter had made yet another poor decision.  She'd lash out and rewrite history in an attempt to blame-shift and avoid responsibility.

At the end of the day, I think it's really hard to have a loving relationship with someone who won't take any responsibility for their own decisions and blames you instead.  Like you wrote, it's not fun being a pin cushion.  Maybe you can take it for a while, out of feelings of love, fear, obligation or guilt, in the hopes that she'll stabilize and get the help she needs.  But that's not a foundation for a normal adult relationship, because there are two adults involved, and both need to try in a relationship.  If you feel that all you have done is tried and tried, and yet your daughter won't acknowledge any of your efforts--rather, she RESENTS you for having tried so hard and still BLAMES you for her horrible life--then it's OK to protect yourself.  My guess is that you want her to get therapy more than she does.  You want her to have a stable life more than she does.  You want her to love and protect her child more than she does.  But it seems to me she doesn't want your help.  She wants to do whatever she wants whenever she wants it; she doesn't care if she's being cruel to you or even to her own baby.  So it's OK if you take a break from that negativity.  Let your daughter do what she wants, as long as she bears the consequences, not you.  It doesn't have to be forever.  How about, just start with today.  Give yourself some space and grace.  See how that goes.  I sincerely hope it makes you feel better, if only for today.