That DSS’s mum losing her house isn’t our problem.

Yikes...she sounds like a user to me.
I wouldn't be paying her a penny. Your relationship is with the son and that's the only reason I presume you are still in contact. She's not family, nor a friend I presume. Hence no no no. She should ask her own family and friends to fund her. Or get a different job that does pay the bills.

A woman expected to stand on her own 2 feet? that really is a novel idea but yes I agree with you

I'm in a similar situation. My ds17 and dd20 at uni for another 2 year, da going next year. I've just been made redundant. Looking for a job - it's really hard. I have/had them 50/50 no maintenance, spousal for 5 years that's all which finished 4 years ago. He lives in a mansion drives a porche I live in a small terrace, drive and old banger... he wasn't honest. I'd already rationalised if I lose the house at least the children have him and a roof over their heads. I'd never ask / or expect anything from him. Certainly would not expect my children to support me. Hopefully it won't come to this 🙏

Why on earth should he address it with her? She’s presumably a sentient adult who knows that she, like all of us, is responsible for her own housing and livelihood.

Perhaps your DH should remind her that it will end in 12 months and ask her what her plans are. Universal credit? Second job? Full-time job if she only works part-time now? He should address it with her.

I'm amazed that men can wiggle out of CM that way so easily. I've lived in the US for quite some time, and in some states - not all - the non-resident parent, usually the dad, has to pay the CM or go to jail. Not having a job isn't an excuse. That's the way it should be.

Amazingly, they find a job, any job, and pay their CM instead of going to prison. Funny how that works.

Yes, I think this is my thought.

If she has to downsize to save, where will the DSS stay?

It could be his own place, lots of students can't live home.

Or on the couch, lots do that too.

But if his dad can house him, it seems like he should.

Maybe DSS will have to get a job.

Yes, but the mum has had to put a roof over their heads herself, since I doubt the maintenance was enough for the son and for housing, whereas the OP and her husband have had a joint income. Plus, the mum might have sacrificed earnings when having the DS, if she took time off work and was then a SAHM until her marriage ended.

ETA: Then there's the fact that the cost of living is horrendous these days and has been rising fast for quite a few years now. Did the husband increase his maintenance in line with inflation? It's hard to afford heating, lighting, housing, food, and other necessities these days for single people.

Then there's the fact that maintenance is to maintain the child, so it's an amount which covers things like food, clothes, activities. It's not an amount which covers the parent's expenses, and not things like the boiler breaking down, fixes for things that go wrong in the house, and all the other expenses of life.

She has been managing all this for a long time on one income, unlike the OP who is much luckier.

Even if they're in full-time education? The DSS is at uni. How's he meant to be contributing when he's paying for his education?

Until he's fully independent though, they're kind of a package deal. Technically there's no reason why that should be the case, but when you factor in the human side of things, it is. DSS is unlikely to be OK with seeing his mum suffer.

Darling Step Son … like DIL us daughter in Law and SIL is son in law avd DS us darling daughter and DH us darling husband… and so on

Big assumption that there was a property to split 20 years ago? Perhaps he didn’t buy his first place until later, or he’s living in the property OP owned/inherited.

Or maybe she did get some money and blew it on her business start up?

Not all men are bastards and shaft their exes. Even on MN.

Well, the money is supposed to be for him, not for her - so how is she going to lose her house? Basically you've been funding her. Sod that. Not your problem.

I'm guessing you're giving £1k a month +.

Honestly some people are beyond cheeky.

I assume your husband and your stepsons mother were once a couple with a son, now he is your stepson and both he and his mother are seen as financial liabilities to you. Well legally the contract has ended but that’s not how families work is it? Six more months to enable his mother to keep her home doesn’t sound too much to ask if it’s necessary.

What DH thinks of that - he's got the boy and he's got the decision to make

Or like @Gettingbysomehow she can take some personal responsibility?

So because you were dealt a shit hand she has to have one, is that what you’re saying?

@SoftBalletShoes If custody is 50/50, neither parent is obliged to pay maintenance to the other so the Dad technically didn’t need to pay his ex anything.

He gives his son an allowance plus he gives his ex maintenance.

Yes for part time pocket money , not to contribute to running the house.

I thought it continued until the child had finished full-time education - i.e. if the child goes to uni, it continues until the end of the uni course.

Too bad. She needs to get another job. I had three jobs at one stage in my life when DS was that age as my exH went abroad and didn't pay anything for DS.

@Intervaldrinks I think it’s just the maintenance to his ex that’s stopping?

The Dad pays an allowance directly to his son and also pays maintenance to his ex.

The maintenance to his ex is stopping in December but presumably the allowance will continue until his son finishes uni.

It sounds like your partner might not no longer be financially supporting his son any longer ? Adult kids living at home do still cost quite a lot so maybe it would be fair to contribute to contribute until he is working.
That might be less painful for you than the boy ending up living with you full time.

It’s generous to pay her business debts and when you share custody 50/50.

I suppose it depends on what her plan for 6 months time is. With a child that age, its not impossible she's trying to hold out until she can take a lump sum out of a personal pension, or have some other savings investment mature. Maybe her fixed rate mortgage expired earlier than she expected. Struggling for a few months can hit any of us.
Is it your "problem"? No
Is it fair that your SSs mother has (possibly) born the brunt of the childcare and financial burden for years, and now might lose her home? Some would say "not", others would tell her to suck it up. Only YOU know which side you fall on.