You've basically done nothing but reiterate my initial point yet say you disagree?
My point was that the parent to all of the children should be responsible for contributing equally to ALL of the children. Not having another child with a new partner and then expecting that partner to foot all/majority of the bill for their joint children because he's too busy paying for his existing kids. I don't believe that step parents should just accept that or it be an expectation. The expectation should be that the PARENT doesn't have more children if THEY cannot afford it, not do it but then expect their current partner to pay for it all.
It's so often a theme on here, blaming or putting expectations on step parents which should actually lie with the parents.
Where I don't agree with you is that existing children come before a new child. You talk about step children having no agency as if new children in a relationship do. All children once born are equal, none of them matter more than the other in terms of their PARENTS time and money so yes if that means little Sarah has to reduce her ballet lessons so little Peter can eat then that's what needs to happen. It should have been thought of by the PARENT before they decided to have more kids but neither child had any control over that decision.
What I didn't agree with PP about was that it's a fair expectation that a step parent should just pay more toward any joint children because their partner has more children to pay for. I don't think that's a fair expectation. It may be what happens in reality but it shouldn't be expected imo. If a parent is choosing to have more children with a new spouse it should be on the basis that said parent can afford to equally contribute, like would expect them to with any existing children, or if not possible, at least discussed beforehand, not just expect their new spouse to take on the financial burden once a new child arrives.
I'm the same as PP, I will sometimes pay for more for our son because I want him to have something that DH couldn't afford with having more children. I don't have a problem with that. I'd have a problem though if it was just an expectation with no discussion that I'll pay for everything for our son, a child DH CHOSE to have, because all his financial energy is being poured into his existing children. It was up to him to determine if he felt able to financially provide for 3 children. Thankfully he can but you see it all the time on here where the step parent pays for everything for the joint children and no I don't think that should just be a given.
It's like, if I left my husband and had my son majority time, should I not expect maintenance from him because he already has children from his ex? Of course I should. Because they are all his children and his financial responsibility regardless as to who came first.