How do I stop my step daughter

I think it's lovely that she feels safe with you but agree that it might actually be a bit about being perpetually annoyed with her mother - it's the right age for that!

I'd come up with a middle ground together that feels right to her - maybe Mum in another language if anyone has a connection to another country? Or some sort of special nickname?

Yes, this is the issue, I think. If I thought she was doing it to me to wind her mum up after an argument, then I’d tell her no.

But if she genuinely wants to call you mum in appreciation of your care for her, then that’s a different matter. If relations break down completely with her mum might she end up living with you full time?

I can't believe you would even try to "correct" her! She knows you're not actually her mum. Isn't it lovely that she feels close enough to you to call you mum? She obviously values and needs that close relationship with you. Don't then reject her by correcting her (in other words, telling her you don't want her to call you mum)!

"you basically are my mum"

She's saying this because she's pissed off with her bio mum. Doesn't mean bio mum has done anything wrong at all. The girl is 14 and a ripe age for "nobody understands me" - except you OP! If it wasn't you it would be someone else. You can't correct this in any way that she'll understand, or want to hear, without upsetting her and making her feel stupid. Just let it ride and encourage the "mum" comments to be made in private if possible. I totally understand the grief it would cause if her mum and extended family overheard her.

Take it as a term of endearment.
Don’t hurt her feelings.

Don't correct her. It's what she wants to call you. I had two dads dont make a big issue out of it

I understand you're worried about the fallout but I think you should support her, you are her Mum to her and she's old enough to know what that means for her.

I think it’s sweet and you shouldn’t try and stop her. Tell her Mum if you want to. She’d have to be very insecure to let it bother her. People can have more than one mum or mother figure, it’s a good thing.

I think she's doing it to get back at her mother, you say their relationship has recently deteriorated. Because of this, and as long as she still has any contact with her mother, you need to discourage her from calling you Mum too.

When her relationship with her mother improves she could find herself in a very difficult situation where she doesn't want to call you Mum anymore, but will find it difficult to tell you that. For her sake, I suggest you remind her of how much you love her, but tell her that you don't want her mother to feel hurt and as if she was being replaced.

It would be totally different if she was very small, but she's an angry teen, lashing out at her mother, and bringing you into it

Maybe you can agree a halfway house which isn't "mum" nor is your name - like Mimi or Mama or Maman or something... It seems that she wants you to have a "title" that's more than Janespotatoes, but I also understand your concern about rocking the boat.

After 12 years it is her right to bestow the title of Mom upon you.

I was never allowed to call my step-grandfather grandpa. That man was my grandfather through and through. He refused the title because he was afraid my grandfather would disown my father. I still wish I could have called him Grandpa and I’m a woman in my 50s and he has been dead for a very long time.

What would her mum actually do, it's not clear sorry. Are we talking criminal behaviour? Is your stepdaughter aware that her mum is erm, unreasonable?
I suppose if you are able to talk through openly your worries with her, then she can make the decision with all the information in hand. If you are having to hide information from her (because its not age or situation appropriate) that's fuelling your concern, then I can see how this would be so difficult. Is someone around her likely to report back to the mum if they overheard? That's the impression I got from your OP, sorry if that's not right.

I thought this - like there are a million variations on Grandmother.

Is there anything special she could call you?

Could you together come up with a nickname that she could call you that’s special to the two of you? It would be your jointly agreed equivalent of mum but without the hassle she might get from her mum if she calls you mum too. It would mark out that her relationship with you is special. However worth chatting to her, she might be calling you mum to cement her place in the family if she’s feeling at odds with her mum.

Its her choice, and she is old enough to make it.

Most step parents would just enjoy it.

Ah I think it’s sweet! Children know what they feel and I’d take it as a massive compliment that she feels this was towards you.
DS1 calls his stepdad by his first name, but it’s him he chooses to spend Father’s Day with (he’s an adult, so not like he’s being dragged along), his stepdad who gets bought presents for birthday/Christmas, and he refers to us both as his parents. I fully believe he has a Dad by name, and a different Dad who has the job IYSWIM.

Does she do when talking to you. As in ‘Mum, please help me with this’ or is it in reference to you when talking to others as in ‘I’ll ask my mum’

If it’s the latter then I’d just leave it. My SC did that and it’s just for the take of convenience so they don’t have to spell out their family dynamic. Ie this is ‘expatornot’ and she is my Stepmum.

No advice if it’s the former. I wouldn’t like it but wouldnt know what to say either to stop it.

If you act like a mum and she sees you as one, then IMHO an occasional "come on, I'm not you're mum but I'll try to be a good stand in" but mainly ignoring it seems to me to make sense.

If the "real" mum gets pissed off because she's such a poor mum that her daughter prefers you as a mother figure then that's on her not you.

Say that you love her as a mum but you're not biologically her mum and you don't want to hurt her mother's feelings if she was to know.

Honestly OP its probably because she feels safe with you. Fair play to you for being an amazing stepmum to her and being her safe space while she goes through turbulence with her mum.

That’s a shame. I can see how, if she’s currently annoyed with her mum and is transferring her affections to you, that might be triggering for her mum but you should speak to your SD about this and if she wants to carry on calling you mum, I think it’s fine. I say this as someone who has had x2 step mum’s, one of which I would never have called mum and another when I was far too old to call mum.

I would overlook it for now, 14 is a really tricky age (15,16 are no better) but if you really want her to stop it then encourage it. Tell anyone who will listen how sweet it is, really big it up! There’s nothing more off putting to a teenager than an adult endorsing something.

This child wants to feel connected to you and safe in that important mother/daughter relationship. Its a gift she is giving you and herself. Instead of treating this like a problem or a river to be dammed think of it like a flow of water to be channeled safely, to grow the relationship.

You have already explained to her the problem you both will face if the mother finds out. No need to repeat that message. Why not say (if you feel you can) “Darling, this is a big honour for me and something important to both of us. I am proud and happy you are choosing to call me “mum”. But can we pick a special term, just for is, that means the same to us but is easier for your mum to handle? Whatever you choose will signal “mum” to me even if its “penguins” or “bobs your uncle.”

if that’s what she wants to call you then let her, stopping it sounds like you are rejecting her and the comment about basically being her mum tells you everything about how she feels about both you and her real mum. Sound like her mum hasn’t been there for her but she feels secure with you, don’t risk ruining that for the sake of not hurting the feelings of someone who seems to be letting her down.