To not want SC to move in as adults?

The gov.uk website disagrees with you (at the end of the page).

www.gov.uk/child-maintenance-service

This.

But if living with op, impacts her as well.

Well, at the moment you own the house and you're the highest earner, so you hold the winning hand.
Personally there is no way I'd agree to this, you can see that it will be nothing but grief with two lazy spongers in your home. So why even consider it?
If you get married you would be wise to get legal advice on how you could protect ownership of your house in the future. But consider whether marriage is important, or whether you'd be best staying single.

If they don’t want to move in now then I doubt that’ll change when they’re 18 (unless they think they’ll have no rules because they’re adults). Let them know that you are happy for them to move in/stay over but regardless of age they need to follow house rules and they’ll be expectations for them to be in full time education or work.

Your DP needs to start having conversations about their future goals/plans and make it clear that without some effort from them then their life will not be as enjoyable as it could be. One parent putting the effort in can really make a difference to how children view themselves and their expectations of themselves.

You need to think really carefully about whether you marry and have more DC with this man.

These children have already been ruined by the ex wife's neglectful parenting and total lack of work ethic. Your DH has also been a neglectful parent too. There is no excuse for him not doing better. The DC are clearly headed down a very bad path at present and this situation seems unlikely to change. I suspect you are right to be cynical about the ex's motives in wanting them to suddenly move in with you once there won't be any more benefits coming her way from the children. The whole situation could make your life an utter misery if they did move in and would probably lead to your splitting up.
You absolutely need to speak to your partner right now, @CynthiaT and repeatedly until he understands that what his ex appears to have decided on will not happen.

Ok I agree that it's a good idea to delay the wedding. I can't see that marriage will provide any financial security for you but it would for your partner.
Also think about why you want to be married, for me its the fantasy of the dress and feeling like I've made it in society.
I cant tell from your posts if you feel that your partner is doing enough to protect his children, that's for you to decide ultimately.

As per my previous comment - no one is forcing
kids who don’t want to apply any effort to school / training or education to do so.
There are plenty of kids under 16 who are not attending school, and currently estimated to be 365,000 aged 16-24 yr olds in no education employment or training. There is very little you can do to force a child to stay in education or get a job / apprenticeship. I say this as someone with a SD who failed all their GCSEs, was kicked out of college for not putting any effort in & then subsequently failed their maths and English resits too.
The future is quite terrifying.

"we would love you to live with us while you're at school, but when you're 18 you'll be wanting to be independent, work so that you have your own money, and to be able to house share with your friends.

We'd hate you to be those sad kind of young adults who can't manage without mummy and daddy looking after them, instead of enjoying independent life and doing your own thing"

Ah… I’m sorry your DP has put you in this position. As others have said: you have a DP problem in that case.

talk to him and tell him that you can’t agree with this.

It might be a good idea for your DP to talk with his children and tell them that they can’t live with him / you unless they’re in full-time education or working (in which case they’ll be expected to make a reasonable financial contribution).

But I feel very sorry for those children. Your DP and their mother have been failing them!

If they playing on leaving school and doing no training this is when the money stops from them in benefits, no doubt this is mums idea .. I’d play her at her own game tell them they move in now or not at all .. lay money on it mum says no

That’s for their dad to deal with.

Actually it isn't, it's taken into account. I'd definitely think twice before marrying this guy.

You have a few years before that happens yet, and chances are their thoughts won't be the same when they reach 16.

Also, they can't just "leave school and become influencers". They have to be in some form of education or training until they're 18, so there's that on your side.

It’s easy money until it’s not easy anymore and the kids are out of control, and she can no longer cope. She will be looking to offload. She did the fun part, she won’t be relishing the consequences, which are coming. You are about to be embroiled in years of misery.

Your dp is securing his free pass/house/parent for his dc/retirement/ pension with his proposal to get married. You are locked in then, all the better if you happen to be pregnant as well.

I would run for the hills with just the one child, and thank my lucky stars. If you don’t do it now, at least line up the escape hatch for 1-2 years time when the shit hits the fan.

Red flags everywhere op and you are wisely pre empting the shitshow coming your way.
I would seriously reconsider the relationship with dp .
Is he viewing your house and income as a good future bet and intends to cuckoo his dc into the home as the easy option without your actual mutual consent?
Difficult because he comes with his dc and if you stay together they are your responsibility too.
I would think very carefully here...

There is no point saying these things to 12 year olds. They are not mature enough to understand the implicit but ineffective threat.

If they even bother answering they’ll just say “yeah yeah yeah”- what else are they expected to say? “no sorry SM, I intend to be unemployed and obese and live off your money so that isn’t going to work, I’ll start planning to live elsewhere”?!? Of course not!

Yes, thanks for that, mine was a pretty lazy response! That would be a scenario that makes sense to me.

YA definitely NBU.

I've told my own son, that as soon as he hits 18, I expect him to contribute, and that means he needs to work.

By then he'll be a lot stronger and have a lot more energy than me, and will be far more able to work. I probably won't but I've set the expectation since he was a small child, that 18=adult=feeding himself. If he goes to uni I'd support him, but he's already earning anyway.

I think you tell both of them your rates now, £500 a month per person, plus 1/4 of the housework,1/4 of the shopping, 1/4 of the cooking, 1/4 of the gardening. They'll soon go off the idea.

Also, these kids are 12 and 13. I wouldn’t set any stock by what they say they will do when they are older. It’s honestly not worth stressing over.

Don't get married OP. It's not this big romantic thing the movies and society dresses it up as. It's a financial contract designed to protect women from the negative impact of pregnancy/ childbirth/childrearing on their careers, earning potential and pensions. If you're the breadwinner and own the house you all live in, then marriage is something you have no need of.

Don't give this deadbeat dad half your house, savings and pension when it all inevitably goes tits-up. Stay as you are, then you can see how this current nonsense plays out without saying anything now and if he tries to move them in you'll be able to just say no and he'll have to move out if he wants to live with his adult daughters.

You can see his attitude with how he's already thinking along the lines of what's yours is his. What's he bringing to the table? Two spoiled brats is all. If you want to stay together for now, I'd have no qualms in playing him the way he's played you. Tell him you'd like to extend the engagement, if it's not already open ended and he's starting to talk plans. Just keep pushing it back another few years each time, until DSC are established adults. Then if you're certain you want to tie yourself pointlessly to him for all eternity or have an expensive severance break, you can do it then once DSC are out of the picture. Personally I'd be using their teenage years to get my own life set up to go it alone as a single parent at some point. Have another child if you want, but don't assume you'll be with DP long term. At least you know he CBA to fight you over custody.

I wouldn't be paying for university, flat deposits or anything else for DSC either OP. As they get older your relationship with them is going to fail. They don't like your rules and trying to enforce them as they get older will lead to them not liking you. You don't owe funding to someone else's children. Especially ones who don't like you, aren't going to be grateful for anything you kindly give, because they're entitled in their attitude and expect it as standard and who won't stay in touch with you as adults either. You'll probably only see them once or twice a year on special occasions, they're too lazy to help nurse DP if he needs it in old age and if DP dies before you they're not going to be stepping up to provide old age care for you, you'll probably never see them again. So don't throw your life or money away playing at being parent to these children who aren't yours and don't think of you in that way.