To believe sis(12) over SD(11)?

Because her sister is an important person in her life and something is going on with their mother so that the sister cannot stay there always or "at convenient times". What about all of this is so difficult for you to understand? You sound like the step-daughter (who isn't really a step-daughter since they are not married).

"Sisters before misters." Every time

Could either ones phone have been hacked? And by an adult?

You can't take the word of one over the other, you need to see your sisters phone, and when you know who is telling the truth you can then deal with it.
you can't automatically take your sister's part, if you have an ongoing relationship with your partner. It sounds as if your sister is jealous- although she has often stayed with you it is more normal that you look after your step daughter than your sister, and she clearly realises that- you are putting your head in the sand if you think your sister has no motive-she clearly does. You can't put up with bullying like that- it has to be dealt with.
You have stepped up to look after them both, but that means sorting this out is your responsibility.

True.
Also, we need an emoji that says "RTFT - or at least the updates".

So your sister had said those things to SD?

She didn't send the texts but had said them, it's making a bit more sense as to why SD did it now.

I think this is a reach. It already seemed unlikely the sister sent them from both girls' reactions and the contents of the messages themselves. With both phones unlocked and on the landing, it would be very easy for the step daughter to have sent the messages and is the sort of thing troubled young women can do.

Admitting to a lie that has become untenable and wasn't well thought through in the first place is much more likely than taking the blame for something so sneaky when you were actually the wronged party.

SD has now admitted she sent the messages and lied.

How good at tech are both girls? Google ‘how to retrieve deleted messages on your phone’, modify search depending what phone your sister has. You then at least will see timings of messages etc.
good luck

You know them and I don’t, but I think DSD was originally telling the truth and is now lying.

I think there is a possibility she feels she has told you the truth, you didn’t believe her, and you made it clear you would leave (at least that’s what she heard). I wonder if (in her mind) she is now trying to get whatever acceptance she can get from you by agreeing with what you already told her you believe in the hope you don’t abandon her.

In her mind she is second best - she’s still saying your sister said not nice things to her and you still don’t believe her. In her mind the only version of the truth that you accept is what your sister says and in her mind she cannot trust you to keep her confidences when it comes to your sister (she asked you not to say anything and you did).

She wants you to believe her and she wants to be able to trust you (with her thoughts, fears, and heart) even if that means she has to lie to get you to believe her and accept her. She’s scared of losing you and (in her mind) she will now lose you if she tells the truth.

But like I said, I don’t know them.

If that is the case. Great.
I am always focused on young women not being used by men looking for childcare.
It is unfortunately a regular on MN.
Happy to be corrected if that is not the case in this instance.
You mind yourself OP.

Are you sure your SD isn't just saying she sent them to herself to put a stop to you trying to find out the truth? Going by what you said about her reaction, why would she react in that way to something she knew she had sent herself? It would be pretty hard to unless she's a top actress. Your sister refusing to show you the messages is pretty telling, tbh. If she knew she hadn't sent them she would have let you look at her phone to prove her innocence straight away.

Edit: typo.

well one of them is clearly lying, and from the few responses i've seen, you are on the side of your sister over your step daughter!

if step daughter had sent the messages to herself from your sisters phone, then she would have left the messages on sisters phone, she would have no reason to delete them? sister of course would delete the messages!

if sister found said messages, all hell would have broken loose!

i'd be inclined to believe step daughter on this occasion!

there is clearly a bigger issue at play here! and until you get to the bottom of that, then you are stuck

Isn't he just.
So calculating.
So deliberate.
So manipulative.

God love you OP, you are his skivvy aupair.

"No man as determined, as one to find free childcare for his children".

So true.

Just like "no man more in love, than a man that needs a home".

There is a reason these sayings get repeated again and again.

Her father is failing her. If he's her only stable parent then he needs to stop pissing off for work all the time leaving other people to look after an already abandoned child. Time to look for a new job.

I agree.

I also so wish that we lived in a world where tweens didn't have phones but that ship has sailed! Loads of issues with phone & my SD of a similar age.

So many people haven’t bothered reading all of OPs posts!! The SD has admitted she sent the messages and lied.

I agree she needs more support - from her father.

OP has enough on her plate and needs to prioritise herself and her own family. Her partner should do the same.

This your SD needs reassurance, security and love.

To the poster who said to me 'how dare you' hope you give your head a good knock. It was clear as day it was the SD. Mn is very biased towards SC and against Step parents, I would pack her off to get GP until your dp gets back.

You seem to believe your SD is lying and your sister is telling the truth but not explained why you are so convinced of this. It might seem an unlikely thing for your sis to do but surely it is an unlikely thing for the sd to do too.
I would sit them down together and ask them about it when together. Unless you can prove it one way or another I would not take sides no matter what you privately decide as it will ruin the relationship with the other.
I don't think it is right to send SD to GP - you are either helping to co parent her or not - and you all need to work through this some way.

Was there no replies from your sd to these messages or just a bombardment from your sister?
I do have some sympathy for you wanting it not to be true, I have a similar relationship with my youngest brother.
As previously mentioned, get both of them sat down together, go through both phones and then decide the outcome. I think your partner should be involved in this too.

I'd talk to dp about getting sd counselling.

I guess sd is very jealous when your sister comes to to stay. I would guess it highlights your sister will always be in your life but sd might not be.

I'd keep both girls apart.

Before seeing your update I would have bet it was SD sending them to herself. I've had sds of similar age and sounds exactly like something they would have done to my own children to try and get one over on them.
There is often a LOT of jealousy from step kids, especially girls I'm afraid who often try to get the other kid into trouble as a way to try and gain more attention and love as they often have an absent parent.

It's actually one of the main reasons I'm no longer in a relationship with their parent. The step kids just made so many false accusations (either admitted it or things that had to have been them as the other child actually wasn't present when they claim things to have happened)

Some people have great dynamics with step families/blended etc
However.. There are a lot of issues that people that have never had step kids just wouldn't understand. The dynamics are totally different to a traditional family and the lengths some step kids will go to to try and cause a rift is insane

Your instincts were right, OP and your DSis should not be exposed to this potential and distressing manipulation in future. You must look after her at your place without DSD present. DP needs to take charge and speak to DSD about falsely accusing an innocent party and get her some help/ support. You should also consider that you may be vulnerable to SDS’s lies should she accuse you of something despicable while you are looking after DSD on your own.
DSD needs help but she also needs to understand that actions have consequences. I would put in place the arrangement to look after DSis only at your home without DSD present. That is your priority. DP makes arrangements for DSD when he is away and should not rely solely on you in the future. You should take a step back from DSD’s care and let her parent(s) manage her until you all feel confident the jealousy - or whatever has triggered this - has been worked through and dealt with. You don’t need to withdraw your love or friendship for DSD. Just don’t be solely responsible for her care until this is resolved.