To think that Ex and OW can take a hike when they refer to her as my kids step mum

[Sapphirefling]

Given that both of them have divorced parents who are remarried (one for the third time) and NEITHER ex or OW refer to anyone as a step parent. Ex H has always declared that his fathers wife was 'nothing' to him.
Yet they have asked my children to call her their step mum ? (Which the kids refuse to do thus far, given that they are still devastated by what has happened)

Oh sweet Lord. YANBU in the slightest. I wish it surprised me that there are men out there as pondscum-ish as your XH, but it doesn't. There are too many of them, and I'm sorry you and your DC are being put through this by one of them.

What a fucking moron.

It is interesting that most people are directing the anger toward to woman, do we know that she wants the children to call her this sapphirefling your ex sounds horrid, and it is ridiculous when theyve never met her to expect them to even know that she is more than a friend - its completely irresponsible. But it's your ex's responsibility to do the right thing, no her. Who knows what lines he's spun her...

My step dad isn't married to my mum, but then they've been together 24 years and mum was a widow when they got together.

I still directly call him by his name, but the kids call him grandad.

Forgot to reply to OP, in your case it's totally different and totally inappropriate I feel.

YANBU

I was made to call my stepmother "mum" from a very early age. Messed up.

Its completely inappropriate in this situation for your XH to insist the DCs call her 'step mum'. What an idiot.

I agree to let your DC make up their own minds.

Oh dear what a horrible thing for your children to go through. They haven't even had chance to come to terms with everything. I have a step-daughter but I certainly don't expect her to call me step-mum. I think it far easier on children to let them lead the way but I am perfectly happy with her calling me by name because as far as I am concerned she already has a mum. She did ask me what I wanted to be called though and I said just my name is okay if you want.

Hope your ExH gets his head out of his arse.

Three months

I think by that time in our (quickly serious) DH's children knew that I would potentially one day be their stepmum, but they just used my name rather than using the term stepmum. They did introduce me to others first as dad's girlfriend, then eventually as stepmum long before we married (after 5yrs together) - but the transition was gradual and entirely down to them.

I think by that time in our (quickly serious) RELATIONSHIP DH's children

Sorry missed a word there

Dh and have been together just over three years, married a month. DS had no idea what a step-father is until one of his friends refered to DH as his dad, he said he wasn't and after a little conversation she said 'oh he's your step-dad' - i wasn't there when this conversation took place and so it was a surprise to me to hear him call DH that one day when talking to someone else.
It's not something which comes up very often, the first time Dh used it I was ill and he had to phone in to work to have the day off - when asked why he said he needed to care for his step-son. Then he came and asked me if i minded. My ex has no problem with this either, but then like I say the need to use the term doesn't arise very often.

It's not something either of the 4 adults in his life would ever have brought into his vocabulary, but none of us have a problem with him saying step mum/dad about my DH and ex's OH.

YANBU it's innapropriate for your Ex to suggest and i'm that he even thought it was a good idea to suggest it.