To let my step daughter call me "mum" ?

@Lospecesenelrio I agree with PPs that there is no way you can break this little girls heart. you are the only mother figure she really has. your son needs to learn to be a better person and it sounds like he is used to getting his own way. so probably its time to really lay down your boundaries

Tell him to crack on then because you are not going to upset a little girl who in effect knows no other mother because a man-child doesn't want to share. (ps. he won't call his step mum Mum and it is no reflection on you if he does just to be petty).

Of course you let her call you mum. You are her mum by the looks of things. Poor little pet. Your son is being really mean.

Excellent post. @Lospecesenelrio You do sound like a wonderful mum.

This. He needs to understand, in a calm, firm way, that threats and blackmail won't be tolerated. You love him very much, but his behaviour is unacceptable.

I voted YANBU however the amount of vitriol on here aimed at a very young man is shocking.
Considering most people on mn are happy to pipe up and explain that the adult brain isn't fully formed until people are 25 and we should cut young people some slack....what they mean is we should cut young adult women some slack, boys/men should be thrown to the wolves!

@Lospecesenelrio How do you feel about having young children again & being a grandmother in the next decade ?

You'll basically of had a child at 15, back parenting young children in your 30's/40's then be relied on for social/financial/familial obligations in your late 40's early 50"s via your first child ?

It's going to be quite the long slog of young kids at a time when your current family are teens

OP you sounds like you are a wonderful mum to all four children.

your oldest does sound like a difficult person to live with. Giving you the silent treatment when things don’t go his way is very immature and infuriating.

to be honest as this stage I would simply say

‘I love you very much but I don’t like this behaviour. If you decide to cut me and the children out of your life over your little sister calling me mum then that is your choice. You can live with your dad. I will be here if you change your mind, your little sisters will miss you very much.’

I think you are right - this behaviour has worked in the past so he thinks he can emotionally blackmail you into doing what he wants.

He can do maths! I am sure he knows that you were 15 when he was born. It's fairly clear that it was an accident. He was also an only child for a long time and had a lot of attention I'd say (also sounds like his dad was very hands on which is great).
So it seems quite normal that he'd be jealous when twins arrived and DSD. That's a normal kid reaction in honesty. And sounds like you are quite indulgent of his moods/reactions so this reinforces the response.
I suspect this is less about DSD calling you mum but about his place in the household/world. He's a much loved young man who is no longer a child but is at home a bit like a child. So he's probably just struggling with the emotions of it all. But lacks the emotional maturity to articulate /process it. Again totally normal for a lad of this age!
Maybe the father can help with getting to the bottom of this but I'd wager money on there being something else behind this reaction

This. I’ve got a 23 year old and a 15 year old and if either of them behaved like your son I’d tear a strip off them. His behaviour is pathetic. She doesn’t see her biological mum. To all intents and purposes you are her mum. I’d be speaking very firmly to him about his self centred, selfish behaviour.

This.

OP you need to work out when and why this issue started with your son.

In the same way he isn't a fully developed adult at 19, you weren't a fully equipped parent at 15.

This is so true.

You are to all intents and purposes her mum and should be called that. Her biological mother isn't around and you have stepped into that role. Plenty of step children call a man who isn't their biological father dad - because that is who they are to them.

Your son doesn't sound very mature and there does seem to be some resentment as to the family set up you have now versus the one he was raised in. You can't redo the past and when he matures he will see that you did your best in challenging circumstances. In the same way he isn't a fully developed adult at 19, you weren't a fully equipped parent at 15.

I would call his bluff though as it sounds like he has lots of benefits of your loving home now and this is a lot of posturing. He doesn't get to dictate your relationships with other people and the only person's behaviour he can control is his own.

Reassure him that you love him and that live is not finite - it's not a pie that is divided up.The more people we have to love them the more love we are able to give.

I totally think DSD should be allowed to call you Mum but that is not mutually exclusive with giving DS help and support with how he is obviously struggling. Being told " grow up " isn't going to be enough.

I'd definitely allow her to call you mum.

But I'd try and find out exactly what your 19yo issue is.

To me it seems natural she would. She hasn't seen her birth mum since her siblings were born and will have seen them grow up and start to call you mum. You have been her sole mum in all that time.

And if your ds decides to cut them off that has to be his decision as an adult. Much more than it is to deny a 9yo a choice of name she calls a parent.

I think this might be an important learning experience for your son. He’s obviously insecure, but is asking for something unreasonable and issuing threats when he meets resistance (albeit threats that hold little weight). This is an immature version of the behaviour pattern that abusive men use to control women and he needs to see that it is ineffective, but also that the fears he has that have triggered this behaviour are unfounded.

I think you need to be firm with him. This is not something that affects him or your relationship at all and you need to explain that and say that it will only make a difference between you if he makes it so. You don’t want anything between him and you to change, so then it’s up to him what he does.

He needs to try it for a while and if he can come to you with genuine evidence of the name change affecting his life, then the situation can be discussed again, with a view to preventing the negative impact on him (can’t imagine there will be one, but the door needs to be open).

He cannot be allowed to dictate the way the relationship between you and your step-daughter progresses as that would be an unhealthy dynamic. His feelings need to be taken into account, but he is being unreasonable here and needs, gently and assertively, to be shown that.

19 year olds today seem younger in many ways than I remember being at that age (I had dd1 at 19 with no parental help and was a fully functioning adult). Dd1 is 19 and is in a funny phase of not quite adult, no longer a child, your son sounds like he is in a similar place.

That said at 19 he needs to accept that essentially you have taken over The role of mother to your SD and that if she wants to call you mum it’s not for him to police that. She’s a child who needs to feel secure in her family and he’s an adult who needs guidance to help him act like one and realise that being jealous of a much younger child is not a good look.

I don't think that the posters here who are saying that DS needs support with his emotions, are also suggesting that your DSD shouldn't call you Mum. The two aren't mutually exclusive. It is perfectly possible to allow her this whilst handling it sensitively/supportively with your son. He is still a teenager and the rational part of the brain doesn't fully mature until the age of 25. He is just dipping a toe unto adulthood and is not a grown man in the emotional sense - that would be impossible.

19 year old needs to grow up. Please let your vulnerable as call you mum. She wants to belong and been abandoned by her biological mum. Poor love. Thank you for stepping up and giving her the love and mum care she needs. You are her mum in her eyes. It be cruel to say no to her mum.

You call her your daughter. If you're happy, absolutely let her call you mum. Be firm with him.

This is why men end up being unable to articulate and come to terms with their emotions, because when they do, they are told "oh, just grow up!".

At the age of 19yo your DS hasn't scratched the surface of his adult life. He's at Uni, which is only just starting him out on the road to maturity. He's still living at home, he still has one foot in his childhood and the other in adulthood.

He is mourning his life as it was, this is a big symbolic change for him, witnessing his DSSis claiming a part of you his DM that he doesn't feel she's entitled to. You will always be the first woman in his life, boys can be extremely protective of their DM and he is railing against someone stepping into his perceived territory.

All you can do is give it time, reassurance but not excessively so. Allow him emotional space to process this change and don't apologise for it. its no different to any other change in a young person's life, it needs time to assimilate and integrate it into his world, which will be changing on an almost daily basis as he learns new things and has new experiences.

When he starts dating, it will feel less relevant and he will be focusing on his own life

The advice to tell him to grow tf up is minimalist and inept and will make him resentful and confused.

^^ I just wanted to add that I know most birth mums love their children and the way I put across the child's prospective was not meant to to demean mothers who've heartbreakingly lost their child to adoption in anyway.