To tell her who stole it

I would tell DH that if the items aren’t returned immediately by SD with an apology, you will deal with it by telling SD you know she took the items and the word with her will come from you and will not be done privately. Then once it’s done it won’t be mentioned again. SD needs to own what she has done. Then give your DD the ability to lock her things up.
They are just kids, and kids do stupid stuff, but SD needs to understand there are consequences.

OP does your stepdaughter live mainly with you, 50/50 or weekends? I'm assuming it's probably not as much as 50/50, and therefore she perhaps resents your DD having more time with her Dad than she does.

She should definitely NOT be allowed to get away with this, and should face your DD, and apologise. Obviously this is unlikely to heal things between them, but let's face it, most teen girls don't get on for long without a fall out, even if they are real sisters, so them not getting along has to be expected and dealt with I'm afraid.

I mean, I wouldn’t make my daughter live with another child she didn’t get on well with and who steals from her, for a start.

But I personally wouldn’t even need to think about it, I’d give the other child the opportunity to come clean themselves and return the items but either way I would be telling my DD the truth.

She isn’t going to believe they just magically appeared after everyone looked for them. If mum lies to protect the other child, imagine how she will feel if/when she finds out.

I'm struggling to follow your reasoning here. Your daughter has to be your priority. Much of the focus seems to be on protecting your step daughter and the best way to deal with her.
Your daughter has had her presents stolen which is upsetting. Get the stolen items back and let your SD face the consequences of her actions.
Supporting your daughter has to be your priority. Your husband seems to be prioritising his daughter at the expense of yours.

In that case I’d definitely be sharing this with your daughter, given that at 14, and as the older of the 2, your step daughter needs to experience some consequences for her actions and to take accountability. I would of course be encouraging your daughter to respond with sympathy and understanding if she can and your step daughter is apologetic.

Dad takes your SD to her Mum's and retrieves the items, it's that simple really.

You can then decide how to "reveal" the re-appearance of your daughter's stolen Christmas gifts

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

If your DD is 13, she's not going to believe the magic fairies returned her items is she? 🙄

I bet she already suspects SD of having stolen them; if you return them and DD looks you in the eye and says "where were they?" are you going to lie?

Tell him that his daughter needs to learn not to steal and that you will not tolerate stealing from family. Either everything is returned and this never happens again or not only will you tell your daughter , next time you will tell the police. Theft is theft and who should he think its OK for his daughter to steal from yours like she is more important. Your daughter deserves to know. I hope he has discussed a punishment for his thieving daughter with you. I'm sure he would not be so quick not telling his own daughter had yours been the thief.

How old are both of the children? I think that would massively influence how I chose to proceed.

How would your DP feel if someone had "lifted" something he valued, say his car, for several days?

What would he want the punishment to be then? Especially as it was found, rather than returned willingly; the thief didn't admit their crime.

Does he want his DC growing up they can steal without punishment? Does he not realise it's his role as a parent to teach them that's not acceptable 🤷‍♀️

Obviously, in this instance, it's likely that the thief is expressing displeasure of their step-sibling having something they want (more time with their dad potentially 🤔) how can that be resolved?

@SuperLiliac how did your partner find out they had been taken to her mums house??

I don't see it as hes covering for his daughter because of the fall out, I see it as him hoping the two will get along and worried they won't if jer daughter knows. Hard to know wjat you'd do if you were in their shoes.

Your daughter's trust in you is far more important than her relationship with the thief. I agree their relationship won't be good for a long while but if one was stealing from the other was it that good to start with?

I agree with your husband, I wouldn't tell her as long as you get the stuff back and an explanation and apology, maybe she's not happy or looking for attention or jealous of your daughter, doesn't make it okay to steal but what's the benefit of telling your daughter, they won't be able to move on it and very uuncomfortable in the long run. Do you get on well with the stepdaughter, does she feel comfortable in your house, just curious because something is wrong if shes stealing from you, hope you get it sorted x

This shouldn’t be a secret. She should be made to hand them back and apologise. Give DD a lock for her bedroom door, she needs a safe space and to know you have her back.

Your DH is wrong here, he should be more concerned with why his daughter is stealing from people rather than trying to hide it because of any fall out.

Me too - I knew exactly what you meant!

I think there's clearly a lot of work to do on their relationship and how (un)happy they are sharing a home. However, covering up the theft isn't going to fix that.

So one parent opts out and the other will softly ‘have a word’ and then keep it a secret to protect her. No wonder she’s a tea leaf, nobody can be arsed parenting the poor kid.

Whilst I agree with the principle that the items should be returned with a serious apology, your DH/DP does have a good point about their relationship goign forwards. I have older teens and neither would want to know someone who did this to them. Regardless of apology.

They both ought to be mortified. The fact the mum won’t get involved and the dad is wanting to keep this secret makes me think perhaps she’s allowed to get away with murder with little to no consequence.
Your partner needs to come down hard!