If the financial agreement before were unfair, then he could have refused it. I suspect it was fair. It undermines your position now to be mithering about her over that.
I wonder how their marriage ended? I may get a roasting for this, but what the hell...
My marriage ended because my XH was a cheating arsehole. When we had our child, I started a family with the full expectation that the number of kids we each would be supporting through university or anything else would be under my control. OK, twins, accidental pregnancies... but still, my (our) children and our financial choice.
My XH now has a stepdaughter, and I know that any financial support he makes for his/my daughter will be reduced as a result.
Yeah, my daughter likes her stepsister and that is worth more than £xxxx towards her uni accommodation. Yeah - I know all that.
But am I a bit pissed off inside? Yes. Would I be if it was a half sister? Yes.
Now I wouldn't say anything and I know it doesn't exactly cover me in glory to think those things. But I do.
I think - why the fuck is my daughter getting less because you are an areshole who cheated on me?
Even though your kids are not the result of an affair, if his marriage broke down because of one on his part, or other bad behaviour from him - then I can understand that his XW could still be annoyed that your children exist.
I absolutely agree that the maintenance decision is his, with you. But if I was XW who still had some irritation at an arsehole XH off playing happy families when he fucked up mine, I'd be annoyed at all the "I need to agree it with my wife" stuff.
I'm not saying your husband was the guilty party - I'm just trying to offer up some explanation why the XW could be feeling aggrieved. I know I'm annoyed about my arsehole ex and his new happy family.
Of course, my daughter has no idea!