Sorry that she's been so vile to you, it's such a shame when women let jealousy and malice rule their life and put it before their children.
I'm a step Mum myself with experience in dealing with a horrific ex, who's soul purpose in life seemed to be to make everything as difficult as possible for us in hope to make us split or at least make us unhappy. Don't let her get to you! You are not alone and there are many others on here who have experienced a similar situation.
Unfortunately the rule number 1 is keep yourself distant. You are never to argue with her, this is between your partner and his ex partner. You can never 'argue' with her. Firstly it's not your place but importantly it's not the best interest of the children, yourself or your partner.
I'm sure your partner isn't meaning to 'pamper around her' for her sake. It's just he's worried about not seeing his children. Your partner can be civil with his ex, not over-friendly but civil at least. It's important that you and your partner are able to share with his children that they're going to have a new sibling soon. So you will have to sit down with your partner and discuss how and when to say it (keep it happy and exciting).
Are the children in school? You and your partner can announce to the children and then after your partner drops off the children he could text message her saying that you're having another child, the children know and can she make sure nothing bad is said to the children about it. If she does try to say anything bad, you could ask the school to help your children see it as exciting. It's quite a big change, their father having a new partner of a year, then both their parents about to have more children. The children will need support and the school can help with that.
Don't let her get to you, don't stew on it, don't think about it. Just focus on having the best time with your DSC when they come over. And that's all you can do :)