To tell her who stole it

So her stepsister is a thief and her father cares more about hassle for himself than he does about your daughter's right to her own belongings.

Can you just take them back and return them without even asking the thief?

Then tell your daughter and put a lock on her door as she deserves to know this person is a thief, it could be a credit card next.

Does it matter if she trusts you after you created a blended family with somebody who treats her like an idiot & with other children who steal ?

Surely you could just pull rank & make it clear your relationship with your partner & his daughter is more important then her stuff as that's exactly the set up you've found yourself in ?

Are you having a laugh ?

You don't tell her who stole it you demand it back with an apology by the person (child) who stole it.

You call the house the stuffs being kept at & you ask her mother (who naturally you have good relationship with to be playing blended family's) to please return the stuff ASAP before the next visit

& You never ever let somebody looking for their stuff just think it's disappeared/accuse somebody they can't prove

This 'blended family' sounds like a nightmare

Your loyalty is with your daughter - she has a right to know and make decisions for herself about next steps with all the information. Don’t trade her trust for people who clearly don’t care enough about her.

You really need to have your daughters back here.

Your partners attitude is a serious red flag btw

Your partner is enabling a theft with his attitude of covering it all up.

It needs to be addressed not brushed under the carpet.

Think about the message it sends to both girls.

I agree with every word here. Definitely a lock asap.

I can see why your DH said not to tell her.. I do have sympathy for his perspective as he wants to keep the peace. Deal with SD without causing a rift between the girls. But I do agree with you overall that your daughters trust should be the most important thing to you. If you replace the items you will actively have to gaslight her to avoid her realising SD took them. And that's not OK. So I'm with you. You should just tell her the truth.

Assuming that neither child has any form of developmental delay, SD already knows stealing is wrong and DD already knows who stole her stuff.

Stuff needs to be returned immediately (and replaced at SD's expense if damaged in any way) . SD needs to apologise. DD does not need to accept the apology. You need to stop forcing both girls into a fake pseudo-sibling relationship neither of them want. Her time with her dad is her time with her dad. There's no need for you or your daughter to share it.

Are you married?

It's so utterly unfair on your daughter to inflict these people into her home and life.

Well, she must obviously give them back. And then admit what she did and apologise. It's partly between her and her step sister but that's the only way to move forward. You won't have stealing in the house.
If she refuses to admit it and apologise then you can say she will be finding out from someone else. And the fact you stole with no remorse will incur a worse punishment, so be smart.
It's a shame and needs to be nipped in the bud. I knew I girl who was such a klepto you couldn't leave 50p in a zipped handbag without her delving in given one second's opportunity. Don't let it become a habit.

"My other half doesn't want me to tell my daughter that her step sis has taken them as it will be very difficult for their relationship going forwards."

Your other half is behaving like a fool. Truth will out, either accidentally or deliberately (e.g. SD throwing it in DD's face during an argument). And when it does (when, not if) your daughter's trust in you will be shattered. And she won't think much of your other half either. Frankly, her relationship with you (and even her relationship with your OH) is considerably more important than her relationship with OH's daughter.

Can he really not see that?

It stinks of Disney Dad to me. He wants to be her friend and is unwilling to be a responsible parent. Parenting includes tasks such as pulling your child up for bad behaviour. He has to do this, no matter how much he wants to avoid that confrontation - it's part and parcel of being a parent.

Insist on honesty and a full apology from his daughter.

SD needs to own up to your DD, return the items, apologise, promise this will never happen again, and mean it. Your DD needs to be able to trust you. If you collide in covering this up, she won’t.

Can I ask why you moved two girls who clearly don't get on that well into a home together?

Exactly.

All about the parents, never mind the collateral damage to 2 young vulnerable teenagers.

What’s the alternative, telling her that the things just disappeared? That’s as bad as gaslighting her.

She's a kid, you are an adult acting in her best interests. Sometimes that means not talking about everuything.

This is where you stand up for your daughter. DH can't throw your DD under the bus, whatever his rationale.

DD gets her stuff back.

DSD offers a sincere apology and a promise never to do it again.

DSD gets consequences.

And then the fallout to the relationship lands where it may. DSD created this, not your DD and it shouldn't be hidden from her that her step sister is a thief.

And I don't buy the argument that there's always some unmet need that leads to stealing. Sometimes there is but sometimes it's just greed. You need to find out which.

Very good point.

I wonder if it was your dd who stole from his dd whether he’d be saying let’s not tell his dd.

Or deflect,deny and hope to Christ it all somehow doesn’t get talked about too much
Let your partner and his daughter lie and ask you to collude
YEP there are signs,up to you if you read and act on them. His actions and responses are telling you his priorities

does his daughter have to come to your house?
can he spend time with her elsewhere?
maybe that's something he could arrange.
at least until he's dealt with her behaviour, extracted a proper apology, and she's regained enough trust to be welcome in the home of people she has stolen from?
otherwise he can maintain his relationship with her in other ways/ places.
Put your dd first. If he can't understand why this is important, maybe you should 'un-blend' things.

It’s depressing how people put their wishes to live with a new partner ahead of their children’s wellbeing. Here we have two teenagers who don’t get on having to live (at least part time) under the same roof. My child’s friend (15) has moved out of their mum’s house permanently because mum’s new boyfriend has moved in and they don’t really get on. Honestly what kind of mother would be ok with a partner causing issues with their own children?!

Oh don’t be ridiculous…. The same reason lots of families blend… they believe in their love , relationship and they can make it work

Sometimes it works and sometimes doesn’t - the reason why doesn’t matter it’s what OP does now in the present that matters
OP your relationship with your Dd is the most important and I would hate as a teen to find out my mum knew but hid it from me

Be honest with your daughter.