I’m not sure this is true. It seems that quite a lot of the time, the children of the ‘first family’ are maintained on the basis of the stepparents’ incomes in both houses.
Quite often there’s been a divorce settlement in favour of the now ex-W, especially if she doesn’t have a job. The father is paying child maintenance and trying to house himself (having given up his assets to meet his children’s and ex’s needs). So he finds himself a new woman. This time with a career and, often, assets. A house that can either be used to house his children during contact or sold to provide the deposit for a house that can be used for that purpose (with a mortgage and bills to which she will be contributing equally).
Meanwhile, the children’s mother continues to SAH and finds a man. He will have to pay the bills but this may be offset by him being able to move into a much cheaper house (sometimes mortgage or rent free, depending on the divorce settlement). Regardless, the SC’s lifestyle is being paid for in large part by the stepparents.
In my own case, my husband had no useful (for daily life) assets left because his ex got everything but his pension. He has large maintenance payments to make for his children. And rent is much more expensive than a mortgage (not that he could buy anything because he had no deposit). I have a good career, and (as a result of that) I had assets when I met him. I had a nice house with plenty of equity and an affordable mortgage. Plus the earning power to combine with his and that equity to buy a much bigger house (and much better than what he had when he was with his ex top). He earns more than me but, after he’s paid his child maintenance, he contributes less to the household than I do with my much smaller salary and the child maintenance I receive. I’m changing job soon to a higher paid role, so the difference will be even greater.
What this means is that I am - through having contributed 25% of the value of this house and through contributing more than 50% of the household income (the income that actually comes to this house) - subsidising the SC’s lifestyle to a considerable extent. Or I was, til I left him. So now he’s got more maintenance to pay and is renting a tiny house. And I’ll get screwed over in the divorce settlement to further subsidise the SC (he gets to claim
greater needs despite having not that long ago agreed to a settlement that left him with little but his income). Because the law is stupid.