I'm in the exact same situation. Full time step. DH has full custody. BM hasn't been in the picture for years now. SD11 is rude & disrespectful of both DH and I. Typical pre-teen. As a full timer, I find it impossible to fully disengage but I found that disengaging at the right time and for certain things is the only way I can survive this mess. When it comes to discipline, homework issues, and drama I disengage. Lately I've even learned that I have to stay away from issues I see with DH's parenting style. If I try to make suggestions, it causes more stress to our marriage than we can take so I just stay quiet. Being a full timer is much more difficult in many ways because it's hard to figure out your role. You will never really be their mom, but essentially you ARE. What to do...

This is the type of situation that makes it almost impossible to love and care about someone else's child.

I am also a custodial SM and SAHM (no BM). I do most everything for my SD12 and SD8 as I do my DD12.

Disengaging is not usually an all or none experience. You'll likely find yourself doing some things sometimes, and others things never, and a few things all the time.

What helped me first was I clearly defined for myself (in my head, not shared) what my "hills to die on" were (your triggers that would seriously make you contemplate leaving your marriage). Those "hills to die on" issues need to have your most consistent and fair attention. In my mind, being 100% engaged on these issues is what works for me. Whatever those issues are for you, defend yourself without bitterness, and do not waffle. For me, my hill to die on includes mean-girl behavior from my SD12 towards my DD12 and SD8 and mini-wife behaviors encouraged or endorsed by my DH. I tolerate neither. Ever. I call both behaviors out every single time and will discipline when appropriate, whether my DH is there or not. I have too much self-respect to let any 12-year old child make me or my DD12, or anyone else in my home for that matter, feel unhappy or unsafe in my home.

Next I "let go" of things that bothered me but did not directly affect me or cause harm to my DD12. I completely disengage. My SD12 loves to eat and frequently has third helpings at each meal despite the fact she is not height/weight proportionate anymore (she's heavy). Not my monkey, not my circus. My SD12 doesn't take her daily seasonal allergy medicine which causes her to whine about how watery her eyes are or how runny her nose is. I'm disengaged. I don't remind her that her eyes and nose would feel better if she took her meds. I make sure she has her meds (I check them occasionally so I can replace them before she runs out) but I don't nag her to take them and I simply say "Yes, this year's allergy season is pretty bad" or some other benign non-engaged comment - similar to what I'd say to be polite to a stranger at the grocery store who made the same comment.

The "sometimes" issues are hardest for me. Sometimes I cook. If I'm making something I know everyone will eat, or has enjoyed before, I will cook. If a skid complains in a disrespectful way (they still will on occasion even with tried-and-true meals), I will normally look at the DH and say something similar to, "Well that sounds pretty ungrateful and DD12 would find her meal in the trash if she said that to me" to which the DH usually verbally reprimands the skid. Once in a while he has thrown their food away. I'm engaged to a point (I call out the offending behavior, but let DH address it, or not, as he sees fit) If DH refuses to reign in his child, I don't complain. But then I go back to no cooking for his skids, and DH gets to be 100% responsible - and he gets to deal with his ungrateful children (which bothers him immensely when they do it to him). I start helping with the cooking after I feel the skids have been reigned back in. This issue is getting better (which is why it's in my "sometimes" category) but is still a work in progress. I used to sometimes attend SD12 school performances that DH is unable to make due to work (usually mid-school-day events around the lunch hour. These performances are usually small and few parents make them due to the time frame.) SD12 took great pleasure that she always had a "parent" who prioritized these events and often paraded me around to her friends whose parents were not able to attend. After being paraded around by SD12, SD12 then bumped up her game a notch and scoffed at DD12 that I missed a lunch date with DD12 (I eat with her 2x/month at school for fun) to attend one of SD12s concert instead. I never told DD12 why I cancelled one of our lunch dates (to accommodate SD12s concert, so this was news to DD12). SD12 basically used Mean-girl behavior to make DD12 feel "lesser" so she could feel like she was "more." Mean-girl behavior is an "always engage" issue for me - so I stopped going to any of SD12s mid-day events without DH. Will I go again someday? Maybe. But I haven't missed a lunch date with DD12 since.

Sorry this was so long. But in summary, basically (for me) disengaging is all about setting boundaries for situations and behaviors I can/can't will/won't tolerate - and maintaining as much dignity as possible while doing it (I still have LYS - lose your shit - moments, but I strive to usually avoid that). And for the rest of the time, those "gray areas" where I sort of engage, but not always, I try to be consistent and pleasant about it. I don't want my skids to feel like they grew up in a home where they weren't wanted, but I also make no effort to try to be something I'm not (their mom). Their mom decided for the skids they would be 1/2 orphans (she took her own life) and that their destiny would be to either have no mom, or a step-mom. I can't change that. I see this as my opportunity to try to influence some "good" into their lives, bring some happiness or normalness into their lives but I don't pity them for their lot in life, and I don't sacrifice myself trying to "make it up" to them either.