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

It sounds like your first loyalty is to your sister, and I'd agree with that, especially since it sounds likely that SD is behind the messages. SD can't make demands about who is allowed in your home. I'd make that clear and tell her you want her to feel welcome here, but you can't and won't ban DSis from the house.

After that, it's a matter of keeping a sharp eye on the two of them, locking phones away, and trying to calm the atmosphere between the two of them, if at all possible. Maybe things will improve once SD realises that you won't be so easily manipulated.

Does your sister not have a lock on her phone? Or Face ID?

I think the issue is that the SD is refusing to be in the house if they have the sister to stay, so there are three options, either the SD goes elsewhere when sister stays, or the sister doesn’t come to stay , or the SD has to put up with the sister coming.
OP I wonder what normal texts between them are like, I think you need to check both girls phones for the conversation leading up to this. Do they normally text ? If not how would your sister have SD’s number ? That would mean that SD definitely texted herself from sister’s phone, as obvs she would know her own number.

The hysterical reaction would make me think it was DSD who sent the messages even more.

If she doesn’t want to stay with you whilst sister is there, that’s her choice.

When will her father be back?

Perhaps he needs to deal with it?

From a point of view of not blaming anyone but finding out what is upsetting his daughter.

Maybe he also needs to look into being around more.

If SD sees Op as a mother it might be hard to see Op mothering someone else.

Especially someone who already has a mother who she lives with.

It sounds like you don’t want to push it with your sister because she might not stay with you?

Is your sister upset at all?

I would be if I thought someone lied and took my phone.

It's a really tricky situation.

I feel sorry for the SD if this is her doing and something has compelled her to do that. Obviously her mum not being around will have affected her badly. Maybe she's jealous of your dsis - are you close to her?

But your little sister needs you and she's your blood relation. Try to get them to sit and chat together with you and get to the bottom of it. Obviously one of them is lying and it's not ok to just let that go - whichever of them has done it they have potentially caused a lot of trouble. I wouldn't just be letting it go I don't think.

It seems very weird that the SD would be able to get into your dsis's phone to send messages to herself (surely she has a passcode?) especially as teens are glued to their phones but equally weird that your dsis would do that whilst they're in the same house together.

I think you need to get both girls together, this is so common for girls of this age, nasty messages, trying to untangle who did what, did they really send it, was it themself etc. It sounds as though your sister needs an escape occasionally and trusts you. Does she have anywhere else to go? Because sd does, but it’s her home, not your sister’s. It’s a dilemma, but I’d get them together for a ‘reconciliation’ meeting, then if you can resolve this, YOU have phones/devices overnight.

This is my sticking point. That's her home. It's not the sister's home. I have empathy for the sister, but not to the point of telling my child she can't stay in her own home.

If my DP told me my dd needed to leave her home so his sister could stay he'd be told that it was not, and would never, be happening.

You cannot make a child leave THEIR home to accommodate a guest.

Luckily my DP would never ask that.

DP, DD and I are a family, by choosing to move in to OUR home he chose to be part of our family, and we all treat each other accordingly. He's more than welcome to bring his family to stay - but not welcome to ask dd to leave to do so. Just as I wouldn't ask HIM to leave to accommodate a member of my family.

There are apps that will fake text messages or WhatsApp conversations. You’re sure this isn’t what your DSD has shown you? Have you taken a look at her phone apps and search history?

Surely you see him and her as your family now...

Bizarre because they're not her family, while her sister is.

Even if the SD isn't telling the truth, why should she leave her home so that Op's sister can stay?

I think you need to be really careful about this, your partner could be seriously pissed off if you basically throw his child out when he’s not there and she is telling the truth. Surely you see him and her as your family now, it really comes across that your sister will always come first. What if you find out she has sent the messages? Are you still going to push the ‘victim’ out because you want your sister there?

No matter who did it, I would say that unfortunately they probably both do need to be sat down together. I don't think they realise what's at stake.

One is related to you and the other is related to your partner. The only scenario in which they don't see each other anymore is the end of your relationship. Is that the aim?

I would tell SD that your sister is welcome here any time and that if it makes her uncomfortable then she needs to stay with grandparents.

I would also say that you aren’t going to take sides, you love both of them and they are both welcome to visit and stay over .

Do not make this your problem and get in the middle.

Stay neutral and let them decide if they want to visit or not.

In this situation your sister comes first.
I would be inclined to believer her.
Your partner needs to make alternative arrangements for his child.
How convenient for him that he gets to travel so much with free childcare.
Are you his au pair?
Sounds like you might be.
Will be interesting to see how he reacts to not having free childcare on tap!

Getting to the bottom of those messages is very important.
SD goes to stay with Granny when dad isn't about.
Look out for your sister.

Your mother needs you to care for your sister your SD will have to rub along or go to grandma's

It does sound like it was probably SD.
I think you need to put passwords on both their phones. Don’t accuse either side. It sounds like SD is a bit insecure. Work on that, but make sure she understands she can’t stop your sister staying. If they can’t get along then have your sister less often and SD can stay with grandparents. If you stop your sister staying it sends the wrong message to SD.

I don't agree with PPS who say it's not important who sent the messages, it's very important imo.

I don't agree with the PP who said your SD needs to be your priority.

I think your sister needs you as much, if not more than your SD, and that of course you have a loyalty to her. Your sister will always be in your life, while your SD is your boyfriend's daughter, so there are no guarantees there, you're not married and you haven't adopted her.

This isn't to say that you shouldn't try to sort it out, but if you can't get to the bottom of it, and your SD doesn't want your sister in your shared home, SD could go to her grandparents while your sister visits you. Otherwise your sister is the loser.

The only problem with that is even if SD is making it up and sent them herself.

If you send her away in favour of your sister, in her head your just like her mum who’s not involved. Someone else who doesn’t care about her. Probably why if she did send them that she sent them. To prove to her that you love her and won’t leave her or pick someone else above her.

If they’re really nasty messages, then empathy must be shown to your SD: nasty messages are unacceptable and inevitably make people upset. Encourage her to tell you if she gets any more.

However, I’d also be explaining to SD that your sister will be staying in your home in future. If she sent nasty messages, then it’s important you talk to her about it and make it clear it’s not acceptable. But it wouldn’t be a reason for her not to stay.

It doesn’t really matter who’s telling the truth. What matters is they’re both clear what will happen going forward.

That way, if your SD is doing it herself to stop your sister from staying, she’ll know it won’t work. On the other hand, if SD is being truthful and has begged you not to mention it because your sister is bullying her and lying to you, you’ve made it clear to your sister it mustn’t happen again.

Sister will still say she didn’t send them though.