If someone made false malicious allegations against you what would you do?

I am in a situation where my husbands step daughter and ex boyfriend made allegations about me that I was seeing some man and f_ _ _ _ _ _ some man up the road for his house. She has also made allegations to my 7 year old daughter about a telephone conversation saying that I threated to kill my husbands step daughter. These are false allegations. My husband and I are separated and he wants me to turn up to family functions with her there and expect me to be nice to her when I am the victim of these allegations.