Advice please...

Last month my husband came clean to me about messaging an ex after we got into a fight. I read the messages and they were innocent, but the problem was that he initiated a conversation with her knowing how I feel about her. She's tried several times to split us up, even gone so far as to photoshop pictures of me with other men. This bitch is obsessed with my husband. Anyway, we live in a small town and I knew that sooner or later we were bound to run into her at a grocery store or something. Well the inevitable happened last night. We ran into her with her kid and mom. No big deal. My husband ignored her when she tried to talk to him and told her to f*** off when she tried to hand him her kid. Well this girl told her mother that I split them up after they had already broken up and that my husband was the father of her 3 month old. Excuse me?! This kid looks EXACTLY like her last boyfriend and looks nothing like my husband. I was called a homewrecking whore several times last night and her mother wished death upon myself and my unborn daughter so that her daughter and my husband could be together like they "should be".

Honestly, IDK how to handle this. The girl is demanding a DNA test to "prove" that my husband is the father when I know he isn't and is threatening to have me "taken care of" so her and my husband can raise her child together. Help??