Sister-In-Law trying to ruin my marrige

My Sister In Law is spreading rumors around the family that I have been having an affair with someone I have never even met. I have been lead to believe that she somehow (don't know how) has construed what she claims to be IM conversations with me about this affair and has them printed up. Right now as I write this I have been told she is on her way to show everything she sais she has to my husband. What can I do? I am tierd of this stuff. This is not the first time that she has done things to cause me harm and heartache. I am thinking on the lines of deffermation but don't know what or how to prove it. Can someone help me please?

Spreading Rumors is a common occurrence in our society. The question is does it amount to defamation.

Slander is false spoken statement to a third party that harms the reputation of the person being spoken about. You must prove damages in a court of law.

Your best bet is to speak to an attorney in your area to find out the likelihood of prevailing in court against your sister in law.

Be truthful with your husband. Honesty is the best policy. If you don't know the person your sister in law is talking about tell him that.