I need advice on how to face this situation **Edit**

My mother is really toxic, she's a Christian and always wants to force my sister to go to church. I believe going to church is good but not when it's forced. My sister txted me last night that she's really depressed and having suicidal thoughts (we are 3 sisters but only the yougest who's 16 yrs old lives with our mom) my mom thinks she's trying to manipulate her by saying she's depressed. My sister is not doing good in school, doesn't want to get out her room at all and is very unhappy, well my mom doesn't see any of this! She just argues all day with her and doesn't try to do anything to make things better my sister asked me to not say anything to our mom because every time I try to say something my mom just says "raise ur son, ima raise my daughter & mind ur business" but I came mind my business knowing my sister can commit suicide ! I don't know how to approach her. Any advice is accepted....

Edit* she doesn't let her go out or anything..