Mom wants to kill herself with pills. What am I to do with a woman who is giving up on life?

Have sister who went into my mother bank personal accts and mine, Knows everything I have and my mother. My brotherinlaw did her income tax after death of my father every yr.. My sister threats her. Mother is afraid of sister.Sister lives out of state came in last week to make trouble . I do not talk to sister , brother in law, neice due to my mother so there would be no stress on mother.She claim she has no use for my sister but when she calls she is as sweet as ice. My mother wants peace. Never happen. Now she wants to kill herself with pills. She tried yesterday I stop her and her balance since she had a knee surgury she fell and wounded her foot. What am I to do this women gave up on life. She is 91 yrs old. I do not know how to handle something like this.