I am in a situation where I am the victim in a malicious allegation what can I do?

My husbands step daughter and her ex boyfriend has made malicious allegations about me that are untrue. I have told my husband that are untrue. I refuse to visit my husband when his step daughter is around because I feel uncomfortable and I would explode and look worse. I feel that he does not believe that she is capable of making such allegations. But I know that she is. I feel extremely discusted. This step daughter is 23 years old not a child.