I get that you don’t want, and don’t have to, top up from your income. But the above comment is absurd. “Go and find a partner”?!?!
All this shows is your lack of imagination or understanding of what it’s like to be a single mother. Perhaps if when they split your H had the bulk of responsibility for the children she would have a new partner by now! Perhaps if he hadn’t had the expense of getting married he would have had more money to put towards supporting his children.
It’s really not straightforward from any angle (including, of course, yours) so why try to force it to be so? Although there has to be a system for maintenance etc (despite any system being flawed and the CMS one especially so), it really makes no sense to me why people think things have to to be so binary.
You don’t say much about the ages of your H’s children, the ages of your children, what the set up was between your H and his ex before you and he got together etc etc. but it’s hard to believe that her earning potential and general situation has not been seriously impacted by the end of their relationship and single parenthood (possibly in a way that his is not?)
If their children are happy spending more time at hers and so is she then that’s great. Also great if he is happy to have them more. But perhaps she is worried that a more equal split of time, or sending the children over more often even on an ad hoc basis, would result in your H reducing maintenance which she possibly relies on to pay essential living costs. Perhaps she is worried that it will be somehow used against her, especially now he has a nicer lifestyle.
I imagine if your DH said he wanted to increase his payments from his income that you wouldn’t penalize his lifestyle/your joint lifestyle as a result. So he could help if he chose to - or is that something you would actively refuse to support?
If you and your H wanted to help there are so many ways to humanize the interaction and make it happen if it makes sense (without setting a precedent which would personally damage you financially). Perhaps his ex is being grabby and unpleasant - but have either of you stopped to ask what’s going on with her? Is there some interim/short term help you could give? Saying ‘oh if you can’t afford the children we will take them more’ is not only unkind, it has a load of emotional ramifications for the children and her, but also really doesn’t decrease costs much when she is still keeping a roof over her children’s heads.
Do your SC do extra curricular activities? In which case does your H pay half of those, half of school trips, half of new shoes etc etc plus maintenance. Or is the maintenance meant to cover housing, bills, food and every other expense because unless he is paying a serious amount over CMS then on a lowish income it’s doubtful that what he contributes touches the sides.
I get it’s not ‘your problem’ in one respect, and if you were struggling to get by yourselves that would be one thing. But these are your step children and your children’s siblings. They will (one hopes) be in your life forever. They are learning how adult relationships work from all of you (as are your own children).
No-one’s saying you can’t do anything (eg fancy holiday) unless everyone else can do it too, but there’s a middle ground here. Perhaps if the conversation was less punitive and hysterical, and more compassionate and pragmatic you might find there were some really easy wins to be had.
I read something interesting recently about not focusing on decisions but on outcomes, and then putting steps in place to try and get to the preferred outcome. So perhaps instead of focusing on whether or not you should part with an extra £50/£100 a month right now, you and your H should think about the kind of relationship you want to have with his DC, the kind of relationship you want all the children to have with each other, the way you’d like them to regard you as they get older etc. and then put steps in place to make that happen.
The way you lay out the situation on this thread I’d be very surprised if your H’s children didn’t quickly grow to resent their siblings (or you might prefer the term HALF siblings), and by extension you. Which could make for some unfun teenage years…
You got involved with someone who has children - I can’t imagine why you didn’t think a certain amount of ‘unfairness’ (or compromise) was going to come along with that… suck it up.