I hate my DSD

Narcissists don't appear out of nowhere, no. They are usually the product of traumatic childhoods and unmet needs.

As adults they have to take responsibility for that; as children, the adults around them should do what they can to repair the damage they've done.

So did you both have a plan as to how to help./deal with her?

Thank you, this is what I don't understand. People are acting like this child has had a full on life of trauma, it doesn't sound like that to me. And what right does that give her to go around trying to ruin everyone else's lives? None whatsoever.

Then dont post on a forum designed explicitly to garnish people's opinions.

Oh yes "psychobabble" as the sneering dismissal.

It is interesting that you position the child as someone from whom the OP must rescue her family. Whilst I might agree that OP and the stepdaughter shouldn't be living together, I wouldn't be prepared to let OP see herself as white as the driven snow here. And if I was to mirror your sneering I would ask who needs to be saved from whom, in reality.

She agreed for SD to live with them. Implicit in that is that she understood the parental responsibilities and what they look like. OP has failed badly in living up to the duties and responsibilities that she undertook three years ago.
The reasons for that failure are actually pretty clear: In psychobabble OP has low insight into her own contribution to the dynamic, in everyday terms: OP is quite selfish and really likes things on her own terms.
The fact OP is seeing a therapist for 'trauma' and the SD isn't, can most kindly be met with a massive eyeroll. If I wanted to sneer my response would be "Wow, that's manipulating the narrative, isn't it."

Yes, the 15 year old is obviously very challenging, even 'good' and 'normal' ones are, but she still deserves to be loved, supported, understood, to be part of a family.

I've just remembered that the old lady I mentioned upthread thought her son probably took after his father. It sounds as though perhaps your DSD takes after her mum!

Hope you can find the space for a calmer life soon, OP. Good luck!

Was she at all like this before you moved in together?

So where's her father in all this?

This

You posted on AIBU....

I'm with you op.

I absolutely hate my dps child. He is just an awful, selfish, nasty, manipulative, abusive little cunt.

I have decided not to live with dp because of his child, because he would never be welcome in my house.

I know it's easier said than done, but I would leave, it doesn't mean the end of the relationship, just means you're not living together for a while, this will probably do your dc a favour too, it sounds like a hellish way to live.

Why do so many people expect an adolescent to display more maturity, emotional resilience and selflessness than they do themselves? If their partner displayed awful behaviour they'd be looking for the reasons; a kid who can't even legally buy vapes or play the lottery, whose life has been thrown about by the adults, and the only explanation is that they're just innately evil.

But you haven't? You haven't taken your children away from all this have you? Why aren't you prioritising them? Your OH can't abandon his child because you've had enough of her, she's only 15.

This is a huge cry for help by her OP. Her mother is not a great parent by the sounds of it. She's lived with both parents then been moved away by her mother or her mother has moved away and dumped her on her dad and so she's now living with her father and his new shiny family. She's no one's priority and that is the attention she is desperately craving. She's gone about it badly because she doesn't know what else to do. I understand that you don't have to put with this but her dad does. No one behaves like this because they're happy and coping well, she's desperately unhappy and her father needs to step up and help her through this. 15 is an incredibly difficult age.

But OP you haven't really said what she's actually done, other than (not saying its not serious) the call to social services. It might well be that she's done lots of other awful things but people can't know that without you telling them.

I'm not saying you have any obligation to tell people anything, btw.

Giving opinions is one thing, telling OP others have been through worse is another. Who cares?! She is going through hell herself.

Then perhaps she shouldn't post on an Internet forum that exactly asks for people's opinions.

Maybe the adults who fucked them up were similarly fucked up by their own parents and shouldn’t be blamed for that either 🤷‍♀️

Maybe next time someone writes about their abusive shit of a DP or their violent neighbour we should feel sympathy for the abusers because it’s not their fault they got fucked up after all.

Basically unless you’ve been tortured and physically/mentally abused from birth you know right from wrong at 15. And very very few teens do behave like the OP’s DSD. If it was super common to make false allegations to try to ruin someone’s life then loads of kids would do it.

What has DSD actually done OP?

OP has already said she's considering it, give her a break.