While I know in theory that we need to support SD16 (as well as SS21 and SD23) and also respect UBPDmom and never ever badmouth the other parent, I'm curious how others have put this into practice.
I think we tend to err on the side of taking the high road (with the kids anyway, I've been more of a smart ass than I should to mom when she aims her crazy-making at me on a couple of occasions. I have my own feelings about her that I am also working on - that's how I found this board, trying to recover my Zen.

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Mom loves her kids, but is much more needy than she probably will admit to herself, and the two girls get tied up in knots trying not to hurt her feelings. She is a smart, capable woman who usually looks quite normal, until she gets upset or feels threatened (usually by something that isn't actually a threat, but something her mind has made up for her is a problem) and then she starts spewing guilt and blame. She can be quite venomous to some people, like DH and more recently me, but I think she does try to rein it in with her girls- and she thinks they are more perfect than maybe is good for them.
SS21 seems to be in a place where he loves and acts respectfully to mom, but doesn't spend too much time or energy worrying about her issues. (What's going on inside who knows, he's a stoic kid like my DH, but he seems to be living his own life).
It's the girls who seem quite enmeshed and walk on eggshells keeping mom happy. While my D's (15 & 19) seem to feel safe expressing frustration with their father and his normal, garden variety foibles, and I can laugh and say, yup that's dad, and no seems threatened, SD23 only talks positively about UBPDmom and SD16 will sometimes say her mom was mad about this or that, but also seems to need to talk up mom as really great.
I'd love to hear from other parents and step parents how they have supported their kids to own their own feelings. I would like to get across that Mom's feeling are her job, your feelings are your job, and if you are behaving with love and respect, you are doing the right thing.
Also, if there are things that you've tried that totally blew up in your face, maybe I can avoid that. :-)
BTW- SD23 sees a psychiatrist, and SD16 sees a counselor, so I am hopeful they are already working through some of these issues with a professional. I do want to try to be sure that what we add makes it better not worse.
Also, UBPDmom has been escalating lately, probably because SD16 has been choosing to spend some time living with us. I think she keeps the kids mostly out of it, but it looks like there have been jabs at both of us under the guise of "protecting" the kids. (It usually looks more like protecting her role as "really super awesome mom with the most super awesome kids ever because of course they have the most awesome mom ever in spite of how awful their father is" but that probably isn't a news flash on this board.)