How do you tell a mother-in-law to stop persecuting you because she believes you gave her son AIDS, when in?

fact you do not have it? She does not know that I know he died of AIDS, and I know she has been watching and waiting for me to fall over. I know she will deny he had it, but I know she knew. She has spread the most horrendous rumours about me, and attempted for years to turn my daughters against me, she has even flown thousands of miles to have a clandestine meeting with my family to discredit me (which she does not know that I know about). I know it is because she was expecting me to sucb to the disease and she intended to either blame me, or make out I am a liar if I reveal that is what he died of. He died of pneumonia/cancer which is what was imparted to all, but the underlying disease was AIDS. She is a highly respected woman and people do believe her. I don't know how to get her to stop, but she is destroying my life. She has caused considerable heartache to my children in her attempts to turn them against me, and I am afraid of her cunning. To my face she is very nice