How to handle an irrational mom as a stepmom?

My husbands ex and i usually have a very good relationship. She was at our wedding, we still have family dinners and so forth. Recently my step daughter acted inapproriately with me in public. After I yelled at her she went home & told her mom that she was upset. Now her mother is turning this into a much bigger deal than needed. My step daughter, who is normally at our house from Tues - Friday hasn't been here all week because " we need to have a family meeting". Her mother is completely irrational saying things like "she is a teenager & does nothing wrong" & " She (meaning me) can't talk to her anyway she want"... I love my step daughter very much & her and I have resolved the issue over the phone but now I have this "meeting & I don't know if I should just keep my mouth shut or not? I feel like anything I say to this woman will be thrown back in my face given that she is acting insane over this whole thing? Any thoughts?