I've only been at it for like 2.5 years (of he!!).  TBH I just had to accept she's an absolute mess. Do I hate it? Yes.  Does it bother me? Ehhhh...

I realized I can't fix it, so might as well laugh at it.

It's just like unlearning any "bad habit." It takes time and practice.  You're going to backside and engage now and then when you shouldn't. Don't beat yourself up when you do. Just keep practicing the "not my monkey" mantra. I think it took a few years of "faking it" before I truly felt disengaged.

Well - I'm not a bio mom, so maybe that helps? But at some point it occurred to me that SS is BM's son, not mine, and as much as I think a dog would be a better parent, she has the right to parent as she sees fit, and I have NO RIGHT to say anything about that.  She is very permissive and created a failure to launch situation, as well as teaching SS to lie and manipulate, but there is nothing I can do about her teaching him the family skills of how to get what you want through subterfuge.

All I can do is be a sane and normal person in his life, and hope he someday decides to choose a different path, and support DH in being a better parent than BM (he is, by far) and hope his influence helps SS as well. 

Your issue is that the person you are living with (your DH), is a crappy parent, so you have to live with the effects of his crap parenting.  I do not have to live with it, because a) SS19 doesn't live with us, and b) DH shuts down any crap behavior and calls SS on his stuff. 

I would run for the hills in your situation. 

I just keep repeating to myself 'they're not my kids. They're not my kids. They're not my kids' lol. It's taken some time but I've finally accepted they're really not my responsibility & how they turn out has little to nothing to do with me.

This is a tough one, because I adore my son and am still responsible for my SS19  even though I divorced his father. What I had to accept was that in life, I can't go back and change the past. Whatever happened and how ever ill equipped SS's parents were in raising him, it is not my duty to right that. I can not make up for it and I don't have any way to do it.  Neither should I even try. Precisely because I am not his mother... The mother and father role are played by two people still in his life.

The fact that his mother is generally absent and cannot care or that his father is overly-permissive and more interested in his sister,  who has completely messed up her life, is not something I am responsible for. I can only guide SS in what I think good behaviour is. I can love him and try to give him the same opportunities open to my son. I can teach SS that actions have consequences, set expectations, lead in him in constructive directions, but I can not make him do anything... I can try to form a responsible and decent adult in the time I have with him, but I will not blame myself or feel guilty for the hand dealt to him in the form of his parents.

My son and SS are quite close, they're basically brothers even though they share no biological bond. I have tried to treat SS as I would my own son - there isn't much more I can do - especially because I don't do any more for my own son.   

Being a member of StepTalk and posting/venting here and then listening to other's advice has changed my perspective a lot!  SS17's crappy, manipulative behavior used to set me off every time.  Over the past year I've learned to say over and over in my head "not my kid, not my kid" and take several steps back and remove myself from the situation.  Trying to be supportive of a SO who was a DD crap parent and now can't understand why his kid is a lying, manipulative, spoiled brat is hard.  I no longer give an opinion or advice unless he specifically asks me for it; otherwise my standard answer is "oh SS cancelled on you at the last minute again, that's too bad." I know now that I cannot rip the blinders of my SO's eyes because he wants to keep them on.  He would blame a stranger for his kid's shitty behavior before putting the responsibility on his son.  

You are not in any mom type roll.  Back off.

Chamomile, what ages are your SKs? I have two SSs but they didn't come into my life until they were 18 and 20. We had some bumps because they were raised differently than I raised my own kids. My kids were more responsible and independent than my SSs were when we all became a family. Luckily, my DH was open to seeing the problems and making some changes.  No matter what, I tried to model good citizenship and was firm in how SSs needed to treat me, their dad, my kids, our home. I also always let them know that I/we were there for them as long as their behaviour was acceptable (respect, appreciation, kindness). I would encourage you to try to keep the door open, support them when you can and are willing, but without sacrificing anything you can't afford to sacrifice. My SSs have matured quite a bit and we now have a pretty good relationship. It is possible if everyone involved has the potential and the willingness. If your skids are simply monsters, and/or their Dad isn't willing to step up, there just isn't a lot to work with and you should always save yourself first. 

When I met my DH, his daughters were 5 and 7.  My daughters (who remained with my ExH when I went to live with him), were 21 and 19.    On many occasions in the first few years, I gave DH what I thought would be taken as helpful advice, but he saw it as unhelpful, so I stopped.  I disengaged after about a year in order to save my sanity.  Watching NPD BM bring up her daughters was like a slow car crash.   I don't feel I have any responsibility whatsoever for them.  

I found this a while ago (Im 5 years in this July) and I re-read it every now and then, when I need to remind myself of certain hard truths.

https://blendedfamilyfrappe.com/disengaging-essay

Consider printing this and posting it somewhere they can see it! LOL.

What general advice do you have for separating yourself from that feeling of wanting to right all the wrongs of the seriously dysfunctional BM/BD?

--read this site. It will show you that there are very rare instances where the SO does not parent out of guilt and allow their own kids to use shame to manipulate them.

If you come to any realizations or find something that works, let me know. I am not happy in the present situation, either, and I feel like I've exhausted all but one avenue to change it.

I guess first I would ask myself why I want to step in. If you're getting joy, satisfaction, and find it rewarding, then more power to you. Parenting SD7 is not any of these things to me, so I let her dad handle it. I am available to her as another adult role model, and support her dad as he parents, but I rarely step in and do things for her of my own volition. I do, however, step in if she's being disrespectful to anyone, bc I have basic standards of behavior for the household. But it's not your job to "fix" the stepkids, so again, unless you actually enjoy doing it, I can't imagine that's sustainable.

“What general advice do you have for separating yourself from that feeling of wanting to right all the wrongs of the seriously dysfunctional BM/BD?”

Start by recognizing that feeling is a YOU problem, and that your impulses aren't necessarily correct for the situation.

Kind, giving, people pleaser, likes to help, fixer, facilitator...often these terms are just a cover for codependent tendencies. I know, because realizing I have them really helped me in my marriage, in my disengagement process, and in life as a whole.

Approach the problem by identifying and working on your own issues first. Learn about codependency, apply that knowledge to your own behavior, and you'll be surprised at how many of your step issues resolve.