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

I'm still not sure exactly why you are so sure that your sister is telling the truth and that your SD is lying.

This all happened in DPs and SD’s home where I live 90% of the time but I do have my own place where my sister has always stayed before.

So this is the first time that they have been together?

I think as a pp has put, maybe best to leave SD to her dad & go to your home when he is away in case your sister needs you.

I would make it clear that if your DP isn't there then you won't be looking after SD and she will have to go home to her mother's, as you will be looking after your DS and you need to protect both of them from each other.

Sorry it won't let me edit to add. This is the start of the teen years so you have years of this ahead. Obviously the DC are similar in age but the way you feel towards them is different. Do you want years of this and can you remain fair to both of them?

Sorry yes I missed this in one of the many updates. My advice stands, taking sides when you have no idea would ruin your relationship, as would asking your SD to leave her Dad's house so you can have sleepovers with your sister there. You'd need to change the dynamic and stay at your own house if you don't want to look after SD anymore.

I think for everyone’s benefit this is what needs to happen going forward.

When DP travels he finds alternative care for his DD, that doesn’t involve you moving in. That way you are still available to support your vulnerable sister at your own home.

haven’t read all the responses so sorry if this has already been said

if you SD sent the messages herself, why would she delete them, since doing so is making it look like your sister never sent them?

if it’s on whatsapp, when i delete something it shows on the chat as You deleted this message, this shows as many times as there were messages.

Kids should definitely have password protection on their phones (that is known to their parents) as there is so much scope for kids taking their phones and sending messages, accessing pictures etc

I think you don't want to believe your sister did this, but I think it's pretty clear she did. She was scared to cause chaos, she was scared for you to confront your sister. Can't you see it? Sister has just played you. SD clearly was very brave in telling you someone was bullying her and hurting her and all you have done is immediately not believe her. It's very unlikely she's done this herself and why would she want to get between you like that. Support SD through this otherwise you'll end up teaching her that her voice doesn't matter

Do you know there are apps which can spoof a number so it looks like it's coming from a particular number? Has SD fallen out or had friendship difficulties with her friends? Just wondering if a friend has done this to be nasty to SD and planted your sister as the villan? Also unless it's Snapchat, you will see that messages have been deleted on your sister's phone

If your SD sent them from your Sis phone why would she delete the messages? Surely she’d want the evidence to remain on the phone. It’s a difficult situation but you can’t really know who’s lying. Has either girl got form for dramatics/lies/manipulation etc?

If they came from your sister's number then she obviously sent them. You need to check her phone regardless and stop making this more complicated than you have. Even if she has deleted them, the carrier will have evidence of it.

Check the bloody phone.

Yes, but some posters have suggested the name had been changed on the messages - ie SD had saved a different device's number as the other kid's name.

As you cannot tell who is lying...
Tell them both...
DS is always welcome in the house it is still her home too..
DSD is also welcome in the house, it is still her home also.
These two factors are non-negotiable.

@WiddlinDiddlin
No I think this is very much the wrong thing to do and could make things much worse for both girls as it is incorrect.

It is in fact DSD’s house. Of course she’s welcome there, it goes without saying. She lives there with her father who has to work away a lot.
Whereas DSis is very much the guest in the house I believe. DSis is normally cared for in the OP’s own house at a separate address when she isn’t at her own home where she lives with her mum.

It’s very possible (though not certain) that with the presence of DSis staying in the house and feeling very comfortable there with her big sister looking after her, DSD has become so fearful of losing her security and control as well as her special bond with her SM, that she has felt driven to do this to secure her place. Who can blame her not to react. Did anybody ask her if it was ok with her for the DSis to come and spend days there. There’s a difference between actually living there and being a temporary guest there and this should be properly acknowledged.
OP says that the girls seemed friendly enough but who knows what was really going on in either of their minds as it appears they have both had such a lot to deal with already in their young lives. Security is lacking for both of them so care must be taken with the feelings of both girls.

Both girls need a lot of reassurance and it’s the DP’s basic responsibility to provide that for his DD when he gets home and finally understands the full facts and truth of what has gone wrong here.

To ensure it doesn't happen again, set up passwords / Face ID if possible on both phones and then remove both phones from an accessible place overnight.

I agree. Things are not OK between these girls and one or both may be crying for help. Tread carefully. Sending abusive messages is serious and so is faking this scenario.

What is going on in general with each of them and with their relationship? Would it help to look more broadly at this? If you are right, is this a cry for help from SD, a way of telling you the things she's worried/embarrassed about?

If you still have SD phone why won't you text your sister "hi"

Hopefully sis would respond hi

Then from SDs phone " why did you send those text messages?"

Surely then however your sis replied would give you the answer you were after and you would know.

Add passwords to the phone as it means anyone could access them! If they have Apple Pay etc, that’s really risky.

The fact that you are "99% sure I believe" your sister over your SD is quite revealing.

In reality there is a fairly equal chance they are lying, and I think the burden of proof is on your sister.

Also, have you kept your SD's phone and not your sisters? You should have taken both of them immediately until you figured out a way of getting to the bottom of it. One way might be to look at some of the activity metrics, such as battery life. If your SD stole the phone just to send messages, then battery drain would be minimal. If your sister had the phone, it would be likely she was doing lots of other stuff on the phone at the same time, and battery drain would be greater.

in my experience tween girls can be particularly tricky and tend to tell a story to a slant that paints them in the best possible light. This isn’t to lie but because of the process of brain rewiring. They lack empathy at this stage as well and things can become very brutal and savage. If this situation isn’t handled with a lot of love and care for both girls, it could permanently sever their relationship.

I wouldn’t be automatically believing your sister. Tween girls do and say the most stupid things and lack the ability to see consequences of their actions. My dd was on the receiving end of some brutal behaviour at this age as well as nasty texts. And I saw dd saying things so out of character that she just wouldn’t have said a year or so before. If a girl of this age isn’t being heavily supported they can become rather feral.

I also think in a way it doesn’t matter who sent those messages. It’s some form of cry for help. Keeping the girls apart is the right thing and not having your dsis in the house is what your dsd wants for the time being. So caring for her when her dad isn’t home then sending her to her gps when you need to care for your dsis. I’d explain to both it isn’t a punishment for either. But to tend to each of the girls needs.

Once you have the girls 121, after a period of time, you may find that one of them reveals they lied. For now, it’s important to speak to both of them but I wouldn’t do it together. They need space from one another. They’re too vulnerable to be treated like warring siblings in a safe family unit imo. Things may be said in the heat of the moment that cannot be undone and you could quickly lose control of the situation. I would do it individually and be clear, you love them and there will be no consequence for telling the truth. You want to believe both of them. However, they aren’t both telling the truth. All you want is to be able to support them both as one of them, or perhaps even both of them, is very unhappy.

In my experience, my dd revealed a lot to me at this age just by holding a line of I love you, I won’t be cross, you can always tell me anything.

@AssassinsBlade made a really good point that this may be your dsd finding a way to tell you something she can’t tell you openly.

Either way, get an agreed PIN code on each of the girl’s phones. You can make a note of the pins on your phone so that you always have access to them. They don’t have to be complicated codes.

Check both phones.

Your step daughter can obviously change any phone number to your sister's name to show her name next to a message. You'll need to physically go in and check the number!

Equally your sister will no doubt deny things too.

Good luck getting to the bottom of it!

Assuming you do not manage to trip up DS into revealing she does in fact know what some of the messages said, which, if she did not send them and they were deleted before she could see them, she should not know (and I don't recommend deliberately trying to trip her up!)...

As you cannot tell who is lying...

Tell them both...

DS is always welcome in the house it is still her home too..
DSD is also welcome in the house, it is still her home also.

These two factors are non-negotiable.

Phones are now to be put into a locked box at Xpm each night, which you have.

That way, no one can send anyone nasty messages.
No one can be accused of sending nasty messages.

Neither has any grounds for objection to this, as it protects them both. (So if either DO have an objection to it, thats a good indicator as to who it really was!).

Then move on - draw a line under it. But examine why, potentially, DSD is trying to push DS out, is she feeling insecure/anxious, theres an underlying issue there if that is the case. And of course it could be DS trying to paint DSD in a bad light just as equally... But that is something to observe and evaluate, I don't see that punishments when you cannot be certain who did what, will achieve a positive result.

Believe your sister but also don't say that you do not believe your SD, in that they came to her phone.
Tell them both that the whole thing has upset you and that you have no more time for the incident.
Insist to your sister that if she did send the messages, she is to stop and never again be so mean.

Insist to SD that you do not believe that your sister sent such mean messages and perhaps she has another friend who was playing a very mean trick. Can she think of anyone who might do that?

Call a truce and shake hands and all go out to a movies or eat ice-cream.

Be on the watch for similar behaviour.
SD could be feeling jealous, vulnerable and unsettled.