The problem isn't your SD, it's your wife. Your SD is simply pushing her limits and she isn't going to  stop. Yes, you're right to insist that your wife pay 2/3 of the bills. Why should you pay for an uncontrollable wastrel?

What keeps you there? Why continue to ruin your health for a situation that is not going to get any better? 

I always ask this: where do you see this situation in 5 years, in 10?  Will it be changed or will it have broken you? 

I'm going to do the usual and suggest therapy. But not for you and your wife, just for you. Someone to help you find your footing, to help you learn to mkae your decisions. To help you respect yourself so you can demand and get respect from those who spend your money and crap in your home..literally and figuratively. 

This sounds like a situation that just has gotten worse due to your wife being a big problem..why doesn't SHE work when you are living paycheck to paycheck?

Honestly, her daughter doesn't sound like she should or could be working.. so I would take that off the table.. she should finish her education first.. period.. and work on other life skills.. but we all know that is going to be tough since your lazy,indulgent wife isn't even working herself.

I am not sure why you don't want to get divorced from this situation that seems like there are two people just using you and sucking you dry...

I agree with your mindset of ok with kids living here if they are working hard at school or a job and paying. Your SD is not doing either. She is 18. You have choices to make. I think you need something to change. I would give them an ultimatum. Either she finishes high school or moves out. She's an adult after all. Being held back 2 years in high school is pretty darn bad. I would be livid at having a lazy kid at my house making messes while doing friggen online school and sleeping in til noon every day whilst failing. Not a chance in hell. 

You are the breadwinner (likely own the house?) you have the power here. Kid failing senior year of high school is completely unacceptable and its time they know that. 

Your description of your SD "hurting herself" and cutting her hair and all the other things indicates that she's got Borderline Personality Disorder. She will not progress in life. This is how her life is going to be. My mother was BPD and had a very weak father. Women with BPD usually have very weak parents and get enabled. Your SD isn't going anywhere. Do not buy a house with your DW. In fact, I'd strongly suggest that you tell your DW that you need to live separately until she gets her DD launched. That won't happen as borderlines don't do well at meeting milestones in life, but at least you'll have gotten out of the house and removed your financial support to two people whom you are not financially responsible for. Then when you're living apart you can chose to either still date your DW or just end it as having a borderline in your life, even just on the periphery, is hell on earth. They just do not leave your life. 

I agree with AgedOut that therapy just for you would be a good idea so that you have support and to maybe delve into how you got yourself into such a situation. Borderlines, use people as resources and that's what you are. They usually learn that behaviour from parent who wants to score a "resource," or a sucker so to speak, who will financially support them so that they can go through life not having to step up. Sorry to be blunt, but I agree with ESMOD that you are being sucked dry. I do understand that divorce is easier said than done, but maybe if you do it in stages like living apart you'll see the difference in your life and it'll be easier for you to pull the plug and escape. Borderlines and their enablers are VERY good at convincing their "resources" that they are responsible for forking over the dough and if you don't, you're the asshole. Don't fall for it. It's only words to try to protect their resource. 

Maybe look up BPD. Although it is treatable if the co-morbidity isn't NPD, the prognosis is slim to none. The treatment plan for borderlines is attending intensive therapy almost every day, even every day to start for several hours a day, which makes treatment cost prohibitive. Therapsists who specialize in BPD actually have limited caseloads and their own therapists because even treating them as a professional is quite an ordeal. BPD isn't just the person afflicted, it is that it's a borderline family system, which means that your DW has major issues that resulted in the development and enablement of a borderline child. In other words, your SD is too far gone and your DW has severe enough issues that she's not going to change. She may even be BPD herself. If not, then she's got borderline tendencies that drive her to emotionally cripple her child so that she never leaves her mommy. 

To make a long story short, you are not responsible for these two train wrecks. They're too dysfunctional that counselling won't be enough for either of them. Run fast and run far. 

SD has to go.  Finishing up schooo in June and you help her getting an apartment.  DW has to understand gervDD has to go.  Better to cost a few $ a month then put up with her crap.  You will not need your sister to clean   

It sounds like your SD has some kind of mental/emotional disability, or depression.

Either way, she is not being parented. This situation will not change and you will continue being unhappy. The only person here who wants change is YOU.

Do you really want to buy a house with someone who is such a poor parent? Who doesnt stand by you? Who undermines you? Who lobbies against you with lazy and probably mentally ill SD?

Id run. You sound like a reasonable and nice person. Consider therapy for you. And take care of you.

I would recommend taking some time to think about these issues prior to buying a home together. Discuss things with your partner. She what her thoughts are with regard to her kid. Go from there. 

I would not tolerate what your immature and exploitative DW and her toxic failed family progeny are perpetrating against you.

Split finances immediately. Sell the car that SD had not paid you for, you pay only 1/3 of any household bills.

She is 18 less than a month.  She can grow up between now and then or she can GTF out and finish growing up on her own time and her own dime.  18 and not graduating from HS makes her out or working and paying full rent, 1/3 of utilities,food, etc....

As for your DW ragging on your 10yo nephew, time to call a spade a spade and inform her that she says nothing to your nephew until she can grow up and parent her own failed family spawn effectively.

Stop being an observer in your own life and live your best life. Your SO adheres and delivers to your expectations as your equity life partner or .... move on, live well, and leave this shit show in your past.

Not many want a marriage to end in divorce, but no one should take a back seat in their own life, their marriage, their home, and their life.  

Set and enforce the standards of behavior and standards of performance in your marriage, family, and home. Everyone  adheres or they are delivered the escalating misery inducing consequences for that failure. Their choice.

Live well.  It is what everyone owes themselves, and it is also the best revenge.

Take care of you.

Working quality's in SD. I'm afraid you see what you are getting.  It's DW'S child it's her job to turn SD into a good hard working person,  she failed, and now not even trying. She happy with things the way they are...SD must go.  You  have to find a way. What's going to be impossible. To get SD  out. 

You need Courage to Make changes

Here’s another father going through a difficult situation.

https://www.steptalk.org/forum/parenting/stepbio-fathers-den/nightmare-stepdaughter-279470#google_vignette

Here’s the story I shared on that post:

My aunt—my mother’s foolish sister, who I’ll call Fool—made one of the classic blunders: she married an alcoholic, thinking because he was exciting, he would make a good husband. It wasn’t until after the baby was born that Fool realized, surprise, surprise, she couldn’t make it work. So, she left.

When her daughter, Dena the Dumbhead, was three, Fool married again, this time to a man I’ll call Clueless. Now, Clueless was a hardworking guy who provided them with a comfortable life. Fool dropped down to part-time work while Clueless became the provider, giving Dena everything she could dream of. He sent her to expensive summer camps, the kind where rich kids learned tennis, horseback riding, canoeing, hiking, camping, and even dabbled in theater. Whatever Dena wanted, Clueless bought, trying to give her the kind of life Fool never could have managed on her own.

And how did Fool and Dena repay this man? With utter disrespect. Dena, spoiled beyond belief, started acting out, testing every boundary. And when Clueless, the one who provided everything, tried to discipline her, Fool would swoop in, screaming, “You ain’t the father—leave my child alone!” It was the same story every time. Fool always took Dena's side, defending her no matter how badly she behaved. Clueless, who had given them both a life of comfort and privilege, was constantly undermined by Fool, treated like an outsider in his own home. The thankless skid could do no wrong in Fool’s eyes, and Clueless was left holding the bag for all their indulgences without any authority to keep things in check.

Well, when Dena was nine years old, the trouble truly began. Yes, you read that correctly. Mostly drinking because that’s the kind of kids she gravitated towards, ones whose parents were alcoholics. At age nine, she started doing pot and other drugs. At age ten, she associated with some teens who robbed a store, got caught and she was sent to correctional homes. During this time, Dena caused chaos and drama and manipulated the division between her egg donor and Clueless. Did I tell you that around age 12 Dena the Dumbhead started having sex?

When Dena the Dumbhead was approaching thirteen, Clueless had finally reached his breaking point. He’d had enough of the chaos, the disrespect, and the endless drama. So, he and Fool separated, probably realizing there was no saving a situation where he was constantly undermined. But the most shocking part? Dena's response. She proudly told my mother, “Now that I broke Clueless and Mommy up, I’m never going to let Mommy go.”

It was as if, in her twisted little mind, she’d won some kind of battle, fully aware of the damage she had caused—and completely reveling in it.

Well, the first thing Dumbhead did after breaking up her mom and Clueless was get pregnant. And, of course, she handed the baby off to Fool to raise. There's an old saying, "If you didn’t raise your own kids to be responsible adults, you’ll end up raising your grandkids." And that’s exactly what Fool did—sacrificing the rest of her life to care for Dumbhead’s son.

But let me tell you, if you thought Dumbhead was bad, her son was a full-blown nightmare from Hell. Grandson wasn’t just a handful; he was the result of a dysfunctional lineage passed down, and Fool, true to form, found herself stuck in the same destructive cycle—raising another troubled child in a mess she helped create. By the time Fool hit fifty, she was already suicidal, overwhelmed by the mess her life had become. She’d spent years sacrificing herself for a daughter and grandson who only drained her emotionally and mentally, most definitely financially. By the time she passed at sixty-two, it was from a stroke—brought on, no doubt, by years of stress and neglecting her own health. She wouldn’t take her medication, and on the day, she died, she was in yet another heated argument with her precious grandbaby. In the end, it was the same vicious cycle that finally consumed her. She died alone. Her daughter was too busy getting high to care.

Now, as for Clueless? He finally caught a break. After the disaster with Fool and Dena, he met a wonderful woman at work, and they got married. Unlike his previous life of sending brats to fancy summer camps, he and Mrs. Clueless 2.0 spent their time traveling through Europe, exploring the world together. They shared a love for the great outdoors and even biked across much of Europe and the UK, living the kind of adventurous, fulfilling life Clueless probably never imagined was possible.

Eventually, they settled down in a beautiful area of New York City, where they spent the rest of their days enjoying their hobbies. Both came from wealthy families, so there were no financial struggles, just the freedom to live on their own terms. They never had children, but they found purpose supporting animal shelters and surrounded themselves with a few unusual pets—probably more fulfilling than raising another Dena could ever have been. Clueless finally found the peace he deserved.

Once the trash took itself out of Clueless' life, he was finally able to embrace the true treasures that life had to offer. It’s like the moment he shed that weight, new doors opened up for him. He found happiness, adventure, and a sense of fulfillment he never could’ve had while dragging Fool and Dena along.

 And honestly, this could very well be the case in your situation too. If you decide to leave, you might just find that the real joys in life—peace, freedom, and happiness—are waiting for you on the other side. Sometimes, cutting ties with the toxic, dysfunctional people in your life is the first step toward a better life.

Sometimes, ‘love is not enough.’

It's important to recognize that you cannot help someone who doesn't want help.

Attempting to change individuals who don't acknowledge they have problems or lack the discipline or courage to change is futile.

If your skid is experiencing mental health problems, these issues stem from early caregivers. If the BM isn't providing the necessary support, it suggests dysfunction on her part as well, this is across the board in her life.

Recognizing this dysfunction indicates that you are likely healthier than they are, that’s why the situation bothers you so.

Prioritize your well-being; by freeing yourself from toxicity, you may discover better opportunities awaiting you.

Good Luck!

Thank you, Rags. Your wisdom and kindness are greatly appreciated. The tale of Clueless, Fool, and Dumbass is indeed tragic; however, the silver lining is that Clueless, being the healthiest of the trio, managed to escape and live his best life. All the advice above by all the posters provided is top-notch and extremely helpful.