Accusation

My mother has accused me of having intercourse with my sister. She keeps on threatening to tell my dad and saying he will cut my throat. I have tried to talk reason with her but she dies not listen. My sister does not speak out at all. My uncle(mothers brother) has schizophrenia. This started 2 years ago after she gave birth to my brother. I fear for my life and reputation. I am afraid for my reputation if I ask the local imam. Please give me guidance.