How can I deal with a bipolar sister who threats our mother?

My sister has a diagnosed bipolar disorder and she is treated by a psychiatrist. She takes her meds but is generally difficult to deal with. Lately she is becoming verbally aggressive with our mother, she accused her of all the worst things and ultimately pushed her to move away from her own house (they used to live together). My mother is desperate and doesn't know what to do and so do I. I think she's having a psychotic episode and worried things might get worse. What would you do? Thanks a lot for your help .