To be starting to feel quite resentful of this situation and to have a talk with DH

It is way more than this. The OP is paying half of all the costs of having her step children living with them whilst her husband pays the morher of his children maintenance for children she has that she doesn't look after. IF he paid all the costs of having the children at their house and THEN used his own money to pay maintenance, it might be acceptable. But it is still taking family money away from the family and paying to an ex who is not incurring any cost related to their shared children.

Indirectly you’re paying her. It’s wrong of him to assume you’re okay with this and wrong for him not to stop it. If he wants to financially support her he needs to do it independently of you, plus he also pays more into your kitty.

I think 50/50 of general expenses is ok as family money. But he needs to stop paying his ex. She is lucky she isn’t being asked to him pay for her children herself

If your finances are separate, then you can't really tell him to stop paying his ex. He's doing his best to look after the mother of his children. That's good, he's being decent. Perhaps revisit things when she is well? I wouldn't push him now, especially when it's not joint money he's giving her, it's his own.

You don't seem to resent the money, but rather the assumption that you don't resent it? Is that right? You can try just having a conversation about it, perhaps say that you feel like your contribution to take aways is indirectly funding his ex-wife. I don't think that's true though, really, but I can see why you might see it that way.

Well the OP has outlined the details. Maybe read the post. He is not a high earner. This is impacting significantly on the household.

.....OP ‘being noble’ with no say in the matter by subsidising the whole tribe. DH, ex and Dsc...

And the ex's other children. Is their father contributing?

What is happening at the moment is that DH, and indirectly OP, are contributing to the maintenance of children that are not theirs when that money could be used for the children in their own household.

So he is pay maintenance to the mother of the children who has no child expense out of his income which is a income within your martiage, and then expects you, who isn't the mother of the children, to pay half of the children's expenses out of your income and just asumes this is OK without discussing it with you. No wonder you are feeling taken advantage of. YANBU. Time for a serious conversation.

Maintenance is for the DC not the ex. As the DC are with the OP he is basically just paying his ex an allowance. And this is ONLY possible because the OP is paying for his DC.
so it’s not the DH who is being decent. It’s easy to be decent with someone else’s money.

DH first responsibility is his DC but instead he is forcing the OP to cover the costs of his DC at home so he can still pay his ex some fun money. I call it this because it’s not money to pay for his dc. It’s just money he’s giving her.
where is the father of the other DC the DH is subbing by paying maintenance for? Because the money he is paying isn’t for his DC ad they live with the OP not the ex

Why? Maintenance is for the dc not the ex. DC are with the OP so why is us paying his ex a salary basically. Because it’s not maintenance if she didn’t have the dc

So being noble only extends to the ex wife does it? And relies on the expectation of the OP ‘being noble’ with no say in the matter by subsidising the whole tribe. DH, ex and Dsc

I can see both sides.
I imagine he feels its a bit of a dickish move to stop maintenance when his dc mum is so unwell.
I can understand your frustration over costs.

How much maintenance is he paying?

We had this very similar situation - DH paid DSCs mum an additional month to give her a chance to get finances sorted but then stopped. We cannot afford it and the money was for the kids. DH and I still pay bills and mortgage the same as we used to but he covers DSCs clothes, school dinner money, social stuff. He also pays slightly more to the food budget to pay for them. I was pretty clear from thr beginning that I couldn’t pay half of all the costs for DSC.

this.

You are not at all being unreasonable! Of course your DP should pay to cover his children. Have a look back over the last few months bank statements to work out how much worse off you are. Sit down calmly and explain to Dp you can’t keep doing this and he needs to adjust his house share out of his income or by reducing the child maintenance. Tbh he really should have volunteered this himself. Given him benefit of the doubt here that he’s a decent bloke and you can work it out.
But don’t be subsidising his children at expense of what could be savings for your own children or pensions etc. Think if the shoe was on the other foot - I bet you would have immediately adjusted the payments so you paid more to cover your children. He is being very thoughtless at best and selfish at worst.
Does he do the majority of his children’s care, washing, cooking, school and hobby car drives etc or is that falling to you too? If he is also leaving you to parent his children and play housekeeper I’d be asking to adjust this too!
Don’t be a doormat - nothing good comes from it except a life permanently down trodden

So if this woman has other children which are living with her, has she got a partner? If she has other children then why can't she have the others?
Paying maintenance should stop. Surely she's entitled to benefits, unless she has a partner living with her, but that's not your DP's problem.
Yes, there's a bit of give and take in these situations but your DP is completely taking you for granted. Your subsidising him and his kids.
Time to speak up.

He is prioritising [sic] ex over you.

While I agree that a timeline for ending maintenance needs to be set, this is a really unhelpful framing.

OP's husband likely knows that the best thing for his DC/OP's DSC is for their mum to get back to a stable place in her life and have a home where they can live part time. Centring the children means caring about their mum's wellbeing and ability to care for them among many other things. Op's DH not wanting to make a hard time harder for her doesn't mean he's prioritizing his ex over OP.

OP, it sounds like your DH is in a difficult spot as a parent. Empathizing with that when you ask for a him to reduce or end maintenance in the near future is a better approach than trying to make this about you vs her.

His children are currently living with their father and op. The ex wife has other children that aren't his. Why should op be subsidising those children and her dh ex wife?

I think you should feel thankful that you're with someone so decent. His children are his biggest responsibility and their mother is the most important person in their lives. They come as a package. It's amazing to me how many men don't seem to understand this or care.

Also it's very short-sighted...surely you want their mum to recover so that the children can go back to her?

YANBU. Tell him he needs to increase his contribution and he can fund that by stopping the payments to ex.

He is prioritising ex over you.

It won’t have any impact on benefits but he’s taking money from his kids and his and OP’s shared kids to get to his ex and her other kids. Which is indefensible and insane.

I would have thought this maintenance money his extra is receiving might be affecting her application for benefits (haven’t claimed benefits for some years so am not certain).

It is certainly money that could be going towards the children, which is what it’s intended for.

Id honestly say that it is time for her financial situation to be separated out.

Agree. He is paying her this money at your expense. I would be very impressed and have a discussion. You've been very supportive in putting up with this for 6 months. You cannot be expected to subsidise her and her other children.

DH is still paying their mum maintenance despite them not having lived with her for over 6 months

That is for the children. That should go into your joint account and you should go back to paying what you paid before.

Why haven’t you raised this before if it is 50/50 then yes he needs to pay more and stop maintenance until she can have them back