I hate my DSD

That depends on how much SS have been involved in her life up to now though.
A young teen that has never had any involvement with SS in her family then yes I would think it's a very unusual thing to do also I would see it as a major indication that something is very wrong.

On the other hand if SS have been involved all her young life you could say she is used to them stepping in and sorting everything out, for better or worse and its normal for her to contact them.
Either way it's not normal behaviour for a healthy happy settled child.

But OP doesn't love her, and why should or would she.
And it's fairly clear that the co parenting isn't on an advanced emotional plane of selflessness.
So where now on the child development journey does OP go, that she is not understanding according to your post.

We hear over and over again that if your marriage isn't perfect or it's a bit dull in the bedroom or your partner is fine but a bit boring you should leave. That we deserve happiness, that our children are better living with separated happy parents than parents who have settled for not abuse and generally OK but not perfect and compromised to keep a family together. Children can sense your sex life is not great because there's resentment building up we are told on numerous threads. We deserve all life can offer us. Don't compromise, you only live once, you deserve to be happy.

Well this is the reality for thousands of children, for step parents, for step siblings, for blended families. The feeling of being abandoned manifesting in toxic behaviour, co parenting weaponised, new children prioritised or having to grow up with conflict from step siblings, our daughters put in danger by bringing non related men into their homes, our sons losing male role models or having no consistant boundaries. The reaction of OP to the behaviour of her SD is normal, why wouldn't you think that of someone who has brought that level of disruption to your life. Just because she is the daughter of her husband doesn't mean OP is going to not feel this way, why would she.

Maybe she felt neglected to the point of calling them.

Plenty of unhappy teens don't phone SS, that's really extreme and not within the realms of normal, even for a damaged teen.

She was 13 when she went to live with OP and her Dad.
As anyone with knowledge of child development will explain to you that is a critical stage where it is essential that they feel loved, wanted and secure as well as stable relationships.
From the OPs own posts it's obvious SD has not had any of that as she was moved from her BM to go and live with them so a new school, lost old friendship group, possibly isolated if she hadn't made new friends. On top of whatever went on that she had to leave her home and move in with Dad.
Healthy, happy well adjusted kids don't phone SS for fun.
What they often do is lash out at the people who they perceive are to blame and this might show up in ways that don't make sense.
That doesn't mean "absolving her for her behaviour" but it does mean understanding where that behaviour comes from, letting her know it caused OP a lot of hurt whilst also letting her know they both still love her.
Obviously that takes both parent and step parent having good personal emotional development and being able to step back and hate the behaviour and not the child or young person in this case.

Well if your (this one’s an adult) child is an abusive little prick then you either have to stay single for life or accept that any partner you have is very unlikely to love your child.

My stepdads eldest child (adult) was absolutely awful and he told us from the off that he never expected my mum or any of us to have anything to do with him, ever, but that he would still see him. The son was beyond help and a seasoned criminal. If my stepdad was insistent that any partner loved his son, he’d have been on his own for life. Nobody would (or should) love that guy.

Really? A pretty massive number of children end up hating their bio parents though - the number of people who are NC with their parents is large. It’s not socially acceptable to say you hate your child but there’s plenty of threads on here where people have said that. Biology doesn’t guarantee love.

Then if he has an ounce of integrity, he'd find an alternative partner.

Let's be honest: not many bio parents end up hating their dc.

It's a decision that needs to be taken very seriously and requires exceptional individuals to pull it off - which is why I'd never move myself into that role.

What a ridiculous thing to say, she's 15 not 5. You can't absolve her from any and all responsibility for her behaviour.

She hates her now after years of horrible behaviour. She didn’t hate her at the outset.

Depends on how awful your child is. He might not have much choice.

There is actually quite a range of lifestyles in between being a monk and inflicting someone who HATES your dc ( in angry caps) on them, in their very home, for your own benefit.

Parenthood comes with responsibility and sacrifice. The problem is way too many people don't seem to grasp that.

14 when we started dating, 15 when he decided to do all the shit he was doing and I cut contact, and is 20ish now.

How old is the child?

Neither would I, I can't imagine not being able to talk about my child with my dp or anything like that, but dp made his choice and it's worked for the last 5 years. We don't live together and won't have kids or anything so we are basically just long term dating.

I'm not judging you - but I am judging him. There is no way in hell I would ever be in relationship with someone who hated my child.

You can't have everything you want to be true be true here - it doesn't hold together. If you think she is an adult who understood the full implications of calling social services and everything that would mean - then that means she desperately desperately doesn't want to live her "incredible life" with you and her father despite him being the only stable parent and the alternatives being pretty unpleasant. Are you really so void of humanity that you can't look at a 15 year old girl and wonder why she felt so desperate to not be in your home that she called social services and maybe these spurious claims.

No reason you should listen to me but here goes. I've read almost all your posts, not many of the others.

I really hope you feel you have had a chance to ventilate and enough useful support in amongst the everything else to help a bit.

You've got two really sensible ideas, go away for half term with your DC and look at having a separate home from DH for the time being. If you legally separate you can have separate homes with less of a financial penalty even if you hope and intend to reunite one day. You sound completely sensible enough to ignore any feeling of 'can't let SD win'.

Your DH might then be in a better position to ask for Social Services help with his troubled teen. I realise there might not be a lot of help on offer.

Looking in my crystal ball (eg what do I really know? - nothing) it might be the best way to prevent intolerable stress from really damaging your relationship with DH long term. If you're not constantly dealing with the shit you might be able to give him better support and he might feel less conflicted if he's bravely on the frontline while younger DC are safe elsewhere.

Last two pennorth: its good to vent but I think, in the long run, it might not be good for your equanimity to hate SD. If you can you need to detach. I say this from a place of empathy having had a similar issue, but with an adult, this year. I regarded them as family, bent over backwards, no good deed went unpunished.

To those criticizing OP: 15yo boys sometimes physically bully mothers/stepmother and this should be seen as domestic abuse. It is not ok. Domestic abuse is not just physical violence. We don't know what line has been crossed here but there's no reason to assume a 15yo girl is incapable of abusive behaviour. Threatening to or actually carrying out mendacious legal complaints and false allegations to social services about child abuse by a parent towards their DC is serious stuff.

I don't agree that the OP is trying to provoke people. It is actually the other way round with several posters trying to get a rise out of the OP. Don't think she needs to be lectured by them about empathy.

I don't think that there is anything reprehensible about her venting anonymously on here, being honest about her feelings and expressing her anger about her situation. People do this all the time on this site, occasionally about their own DC. But then I understand that a step parent is the devil incarnate and need to be punished for having the audacity to want to be happy.

Well my point is that can a hate filled vitriolic person create a loving home? Because if they can maybe they aren’t actually hate filled and vitriolic and it’s more that they have been pushed to their limits.

The 15y may be a damaged child, desperate for love and attention. And may also be behaving in a way that is deeply unpleasant and toxic. Both can be true. She needs help but is sabotaging attempts to support her and alienating those that care. She has had trauma and her future currently looks bleak. She needs her dad to be consistent and loving but with very firm boundaries.

The OP’s feelings are valid. For some time she probably thought that her own two kids were going to be taken away. That’s damn traumatic too. The worst feeling for a mum. Ofc those feelings trump the feelings of care and love she had for her step daughter. She is human and has clearly reached her limit. The OP could be lovely or evil, but there is nothing to suggest here that she is a bad person. She is clearly a mum at the end of her tether.

You desperately needs some space. Would funds allow for a short-term rental or air BnB? You could tell your kids that their step sister is very stressed and needs some peace and quiet to recover.

I don’t think anything will improve without physical space. Good luck.

No I hope she creates an awful home for her kids 🙄

Teenagers can be every bit as bad as adults, and it's entirely possible that this 15-year-old is truly awful despite OP's best efforts. Or maybe OP is somehow to blame for at least some of it. Either way, I can imagine hating a child who was genuinely horrible to me and had threatened my own children's well-being and the stability of their home.

There are no easy answers. It's a shame she's your own children's half-sibling, because now you're somewhat tethered to her for life, too.