I think you're being treated roughly on here op but I think that lots of us first wives can imagine the same being said about us...
Maybe she's a bitch, maybe she's not. 2 years after 25 years is bloody fast to move on and get engaged and get married. I'm 16 months out of a 22 year relationship (17 year marriage) and have just accepted that my decision to be celibate for the rest of my life was, perhaps, extreme 
I get paid around £1700 a month maintenance. That's the CMS's calculation, not mine. I also work but my kids have had a huge drop in lifestyle because before the split, they had much, much more. I buy nothing for myself and all of the maintenance gets spent on the kids in terms of housing, food, school trips etc. He thinks I'm a money grabbing whore but sadly, I'm the whore who's raising his kids, so he can pay.
Also, if my ex decides to stop paying at 18, our kids will really suffer. It's not generous for him to support them through uni. It's certainly not generous to me but it is his fucking responsibility as a parent. I will pay too.
I imagine his new girlfriend thinks I'm the devil incarnate. They got together after he'd ended the marriage- he'd just neglected to let me know. Poor petal, he'd been miserable for years because I'm am an ice cold, frigid, controlling, money grabbing, bitch-queen dontcha know?
I don't quite understand why you have a problem with her. All she's doing is asking him for more money. He can say no if it's as unreasonable as you think it is. She'll just have to deal with it.
She sounds like she's a mess, though and you're probably going to be dealing with her for years to come. I'd detach. Oh and there's no way I'd marry into this fiasco. The writing really is on the wall.