To let my step daughter call me "mum" ?

Aww OP you sound like a lovely mum of all kinds. Given how little your SD's birth mum is present in her life, it's no wonder to me she wants someone to call mum.

Your son is just incredibly threatened by it, and I think if you put yourself in a teenage mindset it's possible to see it's coming from a great deal of insecurity. As he grows independent, he wants it both ways - he wants to be able to be a 'grown up' but is simultaneously jealous of the closeness you have with your SD and worried about being replaced and becoming less important to you as he gains independence from you. It's not easy to navigate emotions at the best of times. He is not expressing himself well and is being ultimately pretty selfish, but I feel it must be coming from that insecurity so trying to address that is the key.

Are there any issues on the horizon with, eg, rooms at home and how long he can call his home really home going forward? How much time do you have just you and him? Is he maybe craving some mum-and-son attention? I know he should be old enough but maturity can come at different stages to different people and 19 is definitely a vulnerable age in terms of the transitions. It sounds like he's just a bit slow on the uptake that there really IS enough love to go around.

Best of luck OP - you'd hope the parenting would get easier by the time they're grown up but clearly not!

Exactly this

Why do people on MN baby young adults so much.

You are aware that suicide remains the biggest killer of young men in the UK?
This young man needs understanding.

Of course 19 is young - catch a grip!

He's barely out of school! He was a teenager during the Covid years - remember them? When their schooling and their socialising and all of that growing up was completely disrupted??? Do you have zero compassion?

This all day long. Let her call you mum, you’re lovely & caring and sound like a great mum and you care for her where she has essentially been abandoned by her birth mother. If your adult child wants to be this unreasonable that’s his problem but obviously do your best to reassure him he’ll always be special to you he’s your son you love him dearly etc etc & this is just a name to help make little girl feel cared for & part of the family. Maybe arrange monthly 1:1 mother & son time especially to help the issue until he comes round.

Well bully for you.

You need a lesson in compassion yourself - for a confused young man who feels pushed out!!!!

You have absolutely no idea of what my kids have been through, certainly more serious in nature than a situation that involves 2 loving parents and 3 siblings who want to be part of a loving family.

This level of emotional understanding isn't really of an adult nature. 19 isn't that young.

Amazing post by @Devonshiregal , i think this hits the nail on the head.

Tell him to get over himself.

Exactly!

Yes but that’s because he’d be her boyfriend not her son. She wouldn’t be supposed to mother him. It wouldn’t be her role. It is OP’s role and he clearly still needs it.

Empathy isn't a massive factor in the life of a 19 year old man. Nor can they always articulate their feelings.

Mind you, empathy is in short supply on this thread, where supposed parents have called this poster's young adult son, who clearly has issues from his upbringing, horrible names!!!

I think the 19 year old is more mature!!!

I understand that but after a conversation to explain you would think he could understand more about the situation.

I would be shocked if no one matured from 19 onwards but I would still expect an adult of any age to have some emotional maturity to understand this situation, most kids of 13 upwards could probably have this sort of discussion and understand it.

I really agree.

OP's updates make it clear she does tell her son he's loved, does make time for him. Yet he's still threatening tantrums over her little stepdaughter calling her Mum.

It's not demonising to say he's behaving badly and needs to grow up. If an OP was complaining about her nineteen year old boyfriend behaving like this, she wouldn't be getting "he's just a child" advice.

Come back to me when your kids have experienced the same life circumstances as this young man has.

Some people on MN understand that YP aren't fully mature when not even out of their teens.

As long as the OP has done everything she can to reassure him he is not being replaced and that she loves him, what else can she do. A 19 yo should be mature enough to understand why a 9 yo needs that security.

I agree too.

There must be a reason he's take this stance. Is he normally able to communicate his feelings @Lospecesenelrio?
Can you show him you're concerned there are problems under the surface if he's really feeling this way?

As a 63 year old with a step father, I think it's terribly complicated and that your son needs reassurance that he is as important as the other children in the blended family.

My children don't really remember my father. My mother tried to insist they called my stepfather grandad. They were married in 1981, my children born in 1994 and 1998. I was incandescent. He was not my father, therefore was not their grandfather.

Because he clearly isn’t as mature as some 15 or 16 year olds. We all mature at different rates and OP herself said she has made some mistakes in hindsight. I’ve matured masses since I was 19 🤗

I am surprised at the low bar for emotional
maturity held here for a 19 year old adult male with no special needs or developmental delay.

While of course he will always be his mummy’s little boy he is also an adult - he is not a child. At his age a lot of adults are living away from home at university or already in full time employment.

this man is throwing a temper tantrum because he doesn’t want his primary school age step sister to call his mum mum! A child who has grown up without her own mother.

while this young man has always had two parents in his life, and by the sound of it two present step parents.

I would understand if was a nine year old - but he is 19!! The way he is expressing his frustration is surprising for someone of his age. How does he cope with other issues in life? What happens when his girlfriends breaks up with him or he fails an exam or doesn’t get a promotion? Has he ever had counselling to help him regulate his emotions?

If he was 29 or 39, I might agree with you.

19 is still so young!!!

What a horrible young man (sorry). He grew up with the benefit of a mother and he has no empathy for a child who didn’t have that luxury. He sounds incredibly self absorbed. If you deny this child what she wants, which is to recognise you are her maternal care giver in name, not only will you likely cause her trauma but you’ll be making a rod for your own back in terms of your son thinking he’s king of the castle.

Also he is much much older than his SS but still young in the scheme of Life.

Blimey. If they both want OP to be mum, then she is mum.