Your husband doesn't understand you don't want to hear about SD? Then tell him. Spell it out so that he can't not understand.

Devise rules for your home. If you have to tell your SD twice to do something then she loses a treasured toy. You should have taken away the toys that were lying on the floor and made her earn them back. Being disengaged does not mean that there are no consequences...

Disengaging works - but you need the actual parent to step up and parent - or you are not disengaging just complacent.

If SD is breaking the rules your DH needs to enforce them. If she has chores and doesn't complete them your DH needs to set consequences.

As for the passive aggressive nature of her attitude - give it back! Take the things that she did not clean up. Then tell your DH that the items have been put away and you will happily give them back when the room is clean. Put it back on the parent!

And when she smirks at you, smirk back. Huge smile a little wave. When she walks around the house acting as if she has zero rules take that moment to guide your son into completing a chore and make a huge deal. Give him a snack when they normally wouldn't get one. Or hand him $5 bucks. Then smile and wave again.

Thr point is - her father needs to father. And you need to take your place back as the respected adult in the home. While putting a little girl back into her rightful spot. 

I would have been tempted to start vacuuming OVER the presents in front of her!  I remember asking YSS to pick up his expensive baseball equipment that was strewn all over the yard outside.  It was beginning to rain so a leather baseball mitt was not going to fare well.  He had played with these new items for about 5 seconds, then ran indoors to play vid games.  This was when dad had to cut out to do an emergency call for an hour or two and before my complete disengaging but after the two oldest had PASed out.

Me:  "YSS you need to bring in your baseball stuff."

YSS (at the time stb 7): (ignores me like I wasn't even there)

Me: "YSS it's raining and your new baseball stuff will be ruined"

YSS (shrugs not looking up from his vid game)

Me:  "It needs to be done NOW!"

YSS: "You can do it."

Me: "It's not my stuff; pause the game and bring it in now."

YSS: "Maybe after I finish this game."

Me: (pulling plug on vid game) "You'll do it NOW!"

Did I mention that YSS used to literally PRETEND that I wasn't there and slam into me on purpose as though he was walking through air?  I gave him a dose of his own medicine and he stopped.  He KNEW what he was doing.

P.S. Pretty much 99% of YSS's stuff was purchased with MY money because Chef was busy OVERpaying his CS to the Girhippo.  In the end I recouped some of my losses by either donating his stuff or selling it which I knew was inevitable (his older siblings had PASed out and stopped doing visitation after their CPS conspiracy with the Gir). Had to wait a sufficient amount of time though after I was sure YSS had PASed out for good so as not to offend Chef or interfere in his shrine keeping.

First, I would like to commend you for taking the first big step, which is not GAF. That's rough. Now You have to move onto step two, which is doling out consequences, natural or otherwise.

Disengagement isn't the same as complacency. What it is is realizing you have no control over the situation, but you DO have control over the consequences.

Like you already realized, when SD doesn't pick up her toys, you do it and take them away. She takes DS's book without asking, you buy him a locking bookshelf that all his things go in that she can't access. She doesn't do her chores and they don't need to be done right now, you may DH until they get done. Over and over again.

Will it destory your marriage? Maybe. Really, though, all it is doing is destroying it faster than it was being destroyed before. That is the big fear people have - that disengagement will end their marriage. Well, what will engagement do? The same thing, but you'll (general) be driven mad in the process and the lightbulb moment won't be until you've expended all your time and energy.

I don't think your marriage is over or anything, but what I am saying is don't be afraid to get mad at your husband for not parenting when it impacts you, and don't be afraid for doling out natural consequences for when SD acts like a brat. Just make sure that you apply the natural consequences rule equally. If your DS does the same things as SD, he needs to be punished the same. 

I have slowly been disengaging as I realize I have no real control. However, you best believe that my DH and my SSs suffer consequences when their piss-poor behavior impacts me. My DH feels the brunt of it because his "ideal" is we all act as family. When I don't get treated as family, I don't act as family, and that hurts him. It's not a perfect system, but it has prevented a lot of unnecessary arguments and hurt feelings over BS that is really his and BM's responsibility.

But - none of what you wrote says disengaged.

Disengaged would be leaving the room cleaning to DH and SD to take care of.

I would have only vacuumed sons sidside of the room. When SD tattles on you to Daddy, look confused and befuttled. "But DH she wouldn't pick up her stuff that I asked her too. Should I have vacuumed over it? Surely you don't want me to encourage her to ignore authority figures by enabling her? "

It will take time, lots of time.  It took several years - years! - for my DH to get it.  And he still sometimes doesn't.

I never talked about "disengagement" per se, just what I wouldn't stand for and how I was going to handle it. Mostly major things like stopping doing their laundry, making sure living room/kitchen were cleaned, no more paying for extracurriculars if I didn't have a say in attendance or days/times, etc.  I didn't get into the details like if the shared areas weren't cleaned in 24 hours then anything left out was either thrown away or donated and DH had to purchase replacements (yes, this included dishes/serving ware).  Or I wouldn't ever touch older SDs room again unless I could smell it from the hallway (and that did happen several times) and then I would clean-slate it MY way, no discussion.

Keep doing what you are doing with your DH, not getting involved "that's nice" or "that's good" is enough. Continue this.

Your SD is young.  That makes it more difficult.  However, she needs to learn.  I would have addressed the book with her once, clearly.  It happens again, no more discussion you put the book back.  A third time, put away something of hers that she can earn back.  That doesn't work?  Locks.

As for cleaning her side, because they share a room - throw all her stuff into a box/bin then vacuum.  She puts it away, or your DH, not you. 

for your own self preservation and sanity.  Out of earshot of DH make it known to SD that you are in fact in charge.  When she tattles just say "Oh kids say the darndest things!!"  Act surprised.    I had to use self preservation tactics because even with disengaging, I had to look out for myself.  I knew it was only a matter of time before abuse allegations would occur since they practically got away with lying to CPS to "make mommy happy"   Had I not been there to get it reversed as there was ZERO evidence but we all know when it comes to NCP dad, you are guilty until proven innocent, Chef would have been in total hot water.

I would NEVER be with them when Chef wasn't within a 100 yard range.  I did get to do a Nanny 911 on YSS one time.  He was actually BEHAVING and playing independently by the end of the day, then Guilty Daddy Chef came back and YSS instantly reverted to baby-like behaviour.  He never "allowed me the privilege of child care" again.  LOL  Honestly I don't know how you ladies do it it with children of similiar ages to these entitled skids!