Stepping back, removing your self from the relationship, or otherwise disengaging. I read posts on here where that is not just a suggestion but outlined as the only other option for SP's. Implying we are idiots for not seeing how easy that solution is to accomplish.
It's right there in front of us. STEP BACK! Since being involved in your sk'slives isn't working, just remove your self from it. That will fix bm's resentment towards you and the place you hold in the dynamic. That will fix all communication issues between BD and bm. That will show the SK's how much you care and fix their PSA effects! Gosh, why didn't I think of that years ago? Oh that's right, I did. And then the impact on my marriage hit me. It's NOT easy. No part of it is easy. And it often leads to harsh judgment and complications causing bigger issues!
Like it or not, when marriages fail and people move on, they get remarried. That means that SP's become involved. SP's! Those folks who chose to get involved because they love their SO and that includes any children they might have. It doesn't mean they chose to get treated like an insignificant speck, or a 3rd class citizen. So stop all the bs about how they shouldn't be involved! Their Marriage dictates they are and will be involved.
Let's focus on tools and techniques for making it work! Because having lived through the backing up first hand......I am aware of how difficult and often times ill fitted that option is. There has got to be a better way. Because backing off.... is extremely difficult emotionally and negatively impacts relationships, permanently!