Don't live in filth! There is a way to be disengaged but be smart

1. Paper plates and cups. But don't just demand their use, hide everything else. Pack it up and store it. 

2. The main living area. Don't allow the slob to muck it up. Take back your space. If she begins to congregate, chase her back into their room.. Turn up the TV. Play loud music she hates. Sing to yourself. That way there is no chance for her to leave her shit around. And if she does toss it back into her sty of a room.

3. Bathroom. Take everything out but the essentials. One towel. Not extras. Toothbrush. Everything else toss into the sty of her room. 

4. Her room and clothes. Who cares. Her problem. Spray in front of the door for bugs. One day she will leave and you can gut the room!

5. Master bedroom, off limits period. It is your sanctuary.

Don't allow this child to push your limits. And if daddy gets pissed oh well. He can go live in the sty with her! 

I had the cleanliness stress early on in my marriage. DH is a messy person, and I knew that going in, but I didn't really know how hard it would be to adjust to.

SS wasn't really the problem, yes, his room looked like a bomb went off in it, but I just shut his door, who cares. As a young boy, he didn't tend to muck up the bathroom, he had good aim and never left his clothes or towel in there.

But things like dishes - DH is also a binge dish washer, he'd say, "leave them, I'll get to them," but I can't stand a counter piled with dishes. Nothing worse for me than trying to make a cup of coffee with a sink full of dirty dishes, it just ruins my whole day.

I realized at some point that if I wanted sanity, a happy marriage, and a house I could live in, I'd have to do it myself.

The cleaner person tends to believe their way is the "proper" way to keep a house and everyone else should fall in line, but that's not really fair to the others, who may not give two shits about a clean house.

So now, I keep up on the dishes so my kitchen is reasonably clean, I keep the living room, bathroom and our bedroom clean. DH's office is a hot mess, as is his workshop and the garage, but I don't care about those.  SS no longer comes over, so he's a non-factor.

In your situation, I'd clean the kitchen, get the plates and other crap out of SD's room, clean the guinea pig cage because I won't be a party to animal abuse, then I'd close her door and let the rest stay there.  I'd also clean the bathroom as other people's filth there is gross.

Then I'd keep any other rooms clean that I want clean - throw her crap in her room if she leaves it out.

Really - it's much easier than living in filth and hoping they will catch on - they won't. And for me, it was better than fighting over it, losing my marriage, or paying for a maid.

But you decide what you can live with.

No matter how much I loved a man, I wouldn't be able to live in such disgusting conditions as you describe - I just couldn't.  I'd move out and leave them to it.   Just reading your OP made me heave.   

I wouldn’t clean these things myself. Hell no. As you said, you’re not a maid, and there are 2 other people perfectly capable of cleaning up after themselves. But I would insist that SD and DH be responsible for cleaning these areas daily/bi-weekly/weekly, whatever is most logical, and then throw a fit to DH if they didn’t. When i moved in we set chores for skids (at my insistence) and if they don’t do them I ask my SO, Did skids do their chores today? Can you tell them to, please? And he does. He knows it’s easier to make skids do chores than it is dealing with a p*ssed-off Step-girlfriend when the house is disgusting.

I never accepted responsiblity for my Dh's kids.   I still feel I have the right to a clean house, I can tell them, no shoes in living room.  I don't reveiw their homework.  I distinguish between house rules (applicable to all in the house) and kid rules (determined by parent, relating primarily to school, health).   I have the right to tell anyone in my house, clean the bathroom after you use it (assumng the bathroom is shared, to any extent).   If stepkid has his own room, all I say is any food or food related products must be taken back to the kitchen.  If the room is only the stepkids for X days, he must strip the bed, or whatever, but I leave it to DH to explain the rules. 

You have to decide what you can live with. 

I have been there. DH is messy (but getting better!) and SD15 a gross person. Filthy sheets, never did laundry (sometimes her personal stench made me gag), rotting food in her room, never took out trash. I would never allow her to have an animal, ever.

So what I did was decide what I could live with.* I need a clean kitchen and living area. 

So if kitchen was filthy I wouldn't cook. Nothing. No dinner if it was my turn. If it wasn't over the top I would clean up, and everything I would put into a box in the pantry. If they don't care then they must not need it. I also would set the dinner table on top of anything left on it. And if I cook I don't clean. 

Things left over 24 hrs in living room: trash, donation bag or put away in a place I know about. I wouldn't get rid of a laptop or anything like that but socks, sweatshirt, books, pens, paper, sunglasses, plates, etc., gone. They must not care about or need them.

Her room I didn't touch unless it was so awful I could smell it in the hallway. Then I would trash bag it. I don't want bugs or vermin in my home. DH hates that but I don't care. Messy is different from filth and messy I can tolerate, but not mold and bodily fluids dried up. 

I also hired someone to clean the SDs bathroom once a month (only one room, affordable) and charge it ti DH. We had plenty of talks before this about SDs being responsible for their space when we moved in. He never followed through so of course it fell on me. It's also our guest bath so needed to be clean. 

I hired a house cleaner a year ago to clean main living areas every other month (again more affordable than monthly). This has been a huge relief. I ended up paying for this myself unless things are out of control, then DH chips in. 

This has saved my sanity! 

I think people have various degees of what they can live with, but everyone should moderate. The clean person shouldn't be the only one to have to accept a dirty house, or a level of mess...messy people should learn to pick up and help the cleaner person in exchange. 

I personally think it's just rude and inconsiderate to leave messes in the way of other people. It says to me that you don't care if someone else is inconvenienced. 

BTW: I never discussed my tactic with DH but for the bagging of OSDs room. I gave warning that if things were not better I would do it my way and that was it. 

This sounds like a kid who is not going to launch, so unless you're prepared to either be the maid or live in squalor for quite some time, you may want to work on your exit plan.  Is there any way BM will take her back or your husband will parent her?