You clearly haven't read what I've written.
No: and, from what OP has said, that isn't what is happening her anyway. Happy for her to correct if I'm wrong but it sounds like he is making a contribution to the mortgage, not paying the whole thing.
Parents should pay 50% of the cost of raising their children, whether they live with them or no. So the mother of OP's step children, if living alone, would need a one bedroom flat. I would expect the absent parent, if there are two children, to be funding 50% of the difference in cost between a 3 bedroom and a one bedroom property. Plus their half of the children's food costs, the incremental cost of utilities etc that relate to the children, their clothes, uniform, activities, childcare etc. How is this even remotely controversial, that if you create a child you should pay half of the cost of housing and raising them?
Any decent person would do that, and it is shocking and yet another sign of the misogyny in society that the law does not compell this. Even the US is far harder on maintenance than the UK! Compelling payment, and payments that are far higher.
A decent person however, does not need to be compellled by the law to do so. Leaving a relationship is fine. Not providing for your children is not fine, it is despicable. And anybody choosing to create more children with somebody who doesn't even meet their basic moral responsibility to pay for half or their child's upbringing is insane, in my opinion, because there could not be a clearer demonstration of moral failing as a human being. If somebody has no regard for their own children's welfare, what kind of partner would you expect them to be? Or that somehow they would magically have more regard for the children they create subsequently? Mad.
With regard to the specific financial arrangements in this case, it is unusual these days to continue to jointly own a house with an ex-partner, although in some divorce cases this still happens. Generally it's better to divide assets at the point of separation, but sometimes it's not possible still to do that while maintaining stability for the children, whose needs should be prioritised.
Maintenance can be considered - if regularly paid and this can be demonstrated and it's drawn up in a legal agreement - by mortgage providers as part of income. So it might be possible for example for the mother of the older children in this situation to take on the mortgage on her own if an appropriate legal agreement is drawn up stating that their father will continue to pay maintenance until the children are 18 at the current rates (plus uprating for inflation) and that the house will be sold and his share of the equity built up until now released to him at that point. That might be worth exploring for a cleaner financial split. But he should still be paying a sufficient amount to cover his 50% of the cost of raising his children, yes. And factor this necessity into any future financial plans or commitments he is making. That's just doing the very minimum to be a decent parent.