I don't know if there are any easy strategies OP, I tried the gentle approach, the stepping back (as far as I could without being totally unfair to my own children in the same house), the ignoring of ExW, the being nice to ExW, the taking on sometimes weeks of sole childcare of DSDs in the holidays with no notice, no asking... (How you step back in THAT scenario is interesting!)
The only thing I found to make a big difference, is being a bit of a bitch. And I mean that not more in a, well there are some VERY self absorbed people around, DSDs and ExW - and eventually I put myself into the middle and became the bigger bitch!
I'm saying it jokingly, but it was hell, and I'm not really a very assertive or stroppy person. I just totally ignored thinking about the good of everyone, and only focused on the good of me and my kids, and expected DP to either come on board with me or go somewhere else.
If anyone had told me I'd have got to the stage (which was 4 years in... ) I would have been horrified. I still don't like it. I really just want a bit of harmony and I'd love it if any blending had worked. But it didn't.
You can't control anyone else OP. If you've tried being fair to all, and it hasn't worked. Then stop and just be really clear to yourself what will work, and be prepared to walk away.