Here is a link to an overview of defamation law in North Carolina, published by the Digital Media Law Project (2014):
http://www.dmlp.org/legal-guide/north-carolina-defamation-law
Defamation suits are notoriously expensive legal actions to pursue, as illustrated by Mass_Shyster’s story. Unless the demonstrable injury to one’s reputation is severe, there are generally better and cheaper ways to handle lies.
You could consult with an attorney in your area to discuss your options. One option is to have your attorney send a cease and desist letter to your defamers, advising them that legal action is being considered and may be pursued if the lies do not stop and retractions made.
I am surprised that your own family and friends would believe that you murdered your boyfriend’s mother. That would seem to be an accusation that would not easily be believed by anyone, especially since the medical examiner determined the cause was natural - and especially by those who know you best.