Slander ... Advice please?

My dad's family, namely his sister and cousin have never liked me, I dont know why and I gave up caring many years ago. The problem is one cousin persistently tells people I am a bad mother who does not know how to look after my kids. Until now I had no real proof, but tonight I had a phone call, there had been a family row and infront of several witnesses including a paramedic, yet again she said I did not know how to take care of my children, can I do anything about this, these claims are upsetting and unjust. I have 3 wonderful children, 1 is autistic but all 3 are doing fantastic at school they are wonderful kids and I am proud to be their mom, I am so upset at these allegations and need to know what I can do. My mother threatened to report her to age concern as she works for them, and she turned to my disabled mother and said "you just dare and you are dead!" this was after shoving my mother and shouting at her.