Stepping back means handing over the reigns of parenting to your boyfriend. That means that he is the main contact, the main one they come to and he is the one who does the 'job' of parenting - i.e getting their meals, liaising with school on his time, health apts, injuries putting them to bed.
I'm hoping that when the kids stay with you, you mean your boyfriend is present as well? That's the first thing I'd want to look at.
Now you can either make more plans on your own when he has the kids so you're giving him that time and space to do that, or if that feels extreme you can be around but just don't do any of the mummy stuff. You can still have a nice relationship with them, it will just be different. Daddy is the go to parent for big decisions and things they 'need' but you are around to support daddy to look after them....subtle difference. Only you know which option sounds right and appropriate for you, given the amount of time you've been together, how things are with the kids and you etc.
As for dealing with a high conflict ex, if she has social services involved, this is clearly quite complex situation and I would encourage you to try and be as neutral in terms of your comments as possible, as pp has stated, the actions may well be pathetic, but honestly you won't win and friends or get constructive advice on a forum full of mums if you say such things.
Hopefully your boyfriend deals with all communications and leaves you out of having to deal directly with her. Close off your social media if you can, so she can't poke and find out things to compete with you on. You need to make yourself boring in her eyes basically so there is nothing to attack. Remain neutral around the kids and resist the urge to correct things that may be said that feel unfair and untrue. Be positive in your comments to the kids wherever you possibly can, 'that's nice of mummy' for example, but don't feel you have to go overboard doing this, you can just make a sincere comment if it feels natural to do so.
And remember whatever you do, this is about the kids, so even if they say or do something that feels unfair, take a deep breathe and remember it's not their fault, walk away, take a walk, a bath, phone a friend, whatever it takes, but remember none of this is actually about you. High conflict people project a lot and you are a convenient target. I'm sorry to say but it sounds like your boyfriend needs to step up a lot here.
((Also a bit of a tangent - be aware that on this forum you will be criticised if you refer to yourself as a stepmum if you're not married. I know you didn't do this, but there are loads of vultures who lurk on here, so I'm just warning you :D Personally I think this is hypocrisy since loads of people don't even believe in marriage anymore and still have kids, so it you're with someone for like 20 years at what point are you stepmum? but that's a side issue. Just want to get in their before someone less supportive does))
Good luck OP.