How to get ex's new partner to back off?!

Hi all, first ever post. I was wondering if anyone could offer any advice on ex's new partner and how to get her to back off. Ex split from last partner about 3 months ago and told my daughters they'd moved to Canada! Then moved in with this new girl within 6 weeks and had my children stay there all without my knowledge. We've been separated for coming up 2 years with the same 'standard' informal child arrangement (i,e, stays with ex every other weekend Fri night - Sunday) and 2 visits to our house per week also. Anyway, this new partner is really muscling in on my 2 your daughters (5 & 2). she already has hew own daughter age 2 so has no need to be trying to take over mine! they've been together about 8 weeks and they've bought a huge family 7 seater car together (my children are with him 4 nights per month!) and shes been booking trips & days away for them all, buying MY daughter a birthday card with all 3 girls cuddling on the front, her idea to buy my daughter a pet for this xmas and coming up with all these grand plans and ideas (bearing in mind my ex has done the bare minimum with them for the past 2 years and has never ONCE asked for extra time with them in 2 years) and now all of a sudden he's asking for extra time with them, long weekends and week in summer etc. Its too much too soon and manipulating my children with the 'fun' family set up. He's making out I'm the bitter ex who hasn't moved on but it's not like that, I'm happy concentrating on being a good mum and holding down my job. It's not fair on my children all this so soon for starters, plus I feel really threatened. We haven't started the divorce yet but are about to & I am worried about what they're planning. Any advice greatly appreciated. P.s. my ex has no backbone and is a compulsive liar so there is no compassion or respect from him whatsoever and he just wants to keep his new 'perfect set up' sweet!

Why should your ex tell you what he is doing? He has a life too and your children are part of that as much as they are yours.
So what if they have bought a family car, it's probably for the trips and outings they have booked, surely this benefits your children?
Wouldn't you want your children to be loved and accepted by their dads partner? Wouldn't you want them to feel accepted in their second home, to feel loved and protected. Surely you would expect your partner to care for your children if he lived with them?
I'm assuming the pet is staying at their house? if so, then why should this even bother you? If she expected you to take it on then I could totally understand the problem.
It sounds like your ex's new partner has been a good influence on him which has benefited your children.

Look at the positives in all of this. You are their mum and no one can ever replace you. Yes the new step mum can be exciting for children, but they will always need their mummy no matter what. Just focus on being a mother and be grateful that they are being taken care of when you are not there. Things could be a lot worse.