Awesome advice above-I won't reiterated it all.
I found myself in a similar situation this last winter-very frustrating and my own failure to uphold my own limits resulted in severe anxiety issues that nearly sent me to in-care hospitalization (and I am NOT prone to anxiety issues in the first place).
I HIGHLY suggest using the following criteria which have helped me TREMENDOUSLY (suggested by therapist to me):
1) state your requests as concrete actions. Include dates/time frames and specific details.
"I'm not comfortable with the kids meeting. We can rediscuss this at the 3 month mark. I will be looking to see that the relationship between myself and metamour is stable with us able to discuss (list 3-4 important topics) ourselves, not having you be the 'middle man' for us, agreements on rules/expectations for kids so that there won't be issues with discipline AND the first (pick a number) times the kids meet will include all of the adults-not just you and her."
2)Make it an agreement that you don't make decisions on the fly. Anything that needs your input outside of emergencies is given a minimum of 24 hours advance (you are always free to respond sooner if you wish) and if its a difficult topic, you reserve the right to delay your response for up to 72 hours for example.