My step daughters mother

My husband has a 2 1/2 year old daughter from a previous relationship and her mother hates me for no other reason than he married me instead of her. They were not together when I met him, but she feels that I am the homewrecker that stole her man. She refuses to let me be around the child and has even gone so far as to disappear with the child for over a year in order to prevent me from being around her. Now that we have filed for full custody she has suddenly had a change of heart and let's him see her once a month for an hour as long as I don't come. I'm not comfortable with him being around his ex without me as she has made several attempts to split us up before. She even went as far as creating some explicit messages and email them to my father. I don't want to stop him from seeing his daughter, but I don't want him around his ex either. Help!