I think I have to agree with a previous poster, concentrate on your pregnancy and yourself for the moment, for you that’s your main priority.
If his ex tries to contact you or get other people to contact you and send abusive things to you then block them and keep a record of it so you can report it to the police if need be.
Regarding your step children, I would say try to shower them with kindness, if their mum is saying things about you then rise above it, it’s hard when pregnant I know from experience. So for example if your step daughter says something like “mummy says you’re horrible!” Just reply with “that’s a shame your mummy feels that way, but it doesn’t change the way I feel about you DsD, I think you’re awesome!”
That way your step daughter does not see you rising to it and getting annoyed and then reporting that back to her mum and it also shows that whatever ill feelings there are with the adults the kids are made to feel special and not be roped into a slanging match between parents, it also shows that you are more mature by rising above it!
Obviously once the kids are back at their mums you can talk to their father about it and vent but I would also consider that his ex has no real way to hurt you other than what she is doing, it’s petty behaviour and when people like his ex act in such a way, it’s usually down to the green eyed monster, and that’s just sad that she has nothing better to do than to do that and potentially upset her children by doing it too!
I would advise to keep that in mind and rise above it! It’s bloody hard! I know and it’s easy to fly off the handle especially when you’re pregnant and someone is winding you up.
Good luck, and congratulations on your pregnancy xx