My mom is apart of a cult

I think my mom is going crazy. She thinks people are trying to kill her and everyone around her is evil. She’s become extremely superstitious and won’t even eat at restaurants cause she thinks people are trying to poison her. She accused my sister of being a witch. My younger sister is alone with her and my dad and being verbally abused and it’s a bad environment for her mentally. She keeps threatening to k!ll herself. My older siblings and I are off in college and keep hearing about these terrible things happening. What should I do? If something happened to my sister I think I would give up living. Idk what I did to get such shitty parents life is so unfair. I keep living in constant fear that I’m going to wake up one day with a message that something happened to my sister.