Please please dont go out, your house is for you and your husband your step children and your own children, she is not to be in it sullying it. Its your safe haven, theres no place for her, even if your not there. Its simple shes not welcome.
If she really wants to discuss the dates, your husband can meet her at the door and they can go to a coffee shop, pub or somewhere, taking the kids if they like, i wpuldnt recomend it, and as you have offered i would strongly suggest your husband is very firm and takes you up on the offer of you looking after the kids. They could even sit in the car and duscuss it, but she is not to come in your house, please.
It really doesnt sound like its a good idea to not have the agreement in writing, can your husnand take a pen and paper with him, get her to sign it and email her the dates as comfirmation.
On the comment i read it as the comment being about the baby not being your step childrens family, and i waa upset by Robos comment that seemed to be putting the blame on to you, thats why i said what i did, that if ot was the way robo thought then no it was asking a little too much, but of it was the way i thought and is, ie disgusting and undermining of course your child is your step childrens family.
I know of grandparents that keep undermining a half sibling rekationship, bly tellimg the older children the younger children are only "half" siblings, some ex families are just nasty. I on the other hand dont make any difference between full half or step their still all family and just siblings
On the comment i think theres three things you can do.
1)let it go, which i understand isnt really possiable, might be best for the kids, bit it does affect everything
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be angry, this plays in to her hands she wants you to come over as the evil step mum
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i actally think doing what fun does is ok, limiting your own contact with her, civil in front of the children but let your husband deal with her, no more contact with you or your child than her dropping the children at the door. If even that let your husband do that, you dont need to speak to her. But not making the atmosphere nasty for the children