Can I Get Temporary Custody Over My Sister

My question involves a child custody case from the State of: Missouri

If you ask your parents and they say "Yes", you can care for your sister. Anything else depends on the facts, but the answer is "probably not".

My question involves a child custody case from the State of: Missouri

I am 22 years old and atrending college with a stable full time job.
I need to know if there is any possibility if me gaining temporary legal custody over my 11 year old sister.
My mother has a heavy history of eveictions, unemployment, and has debts that prevent her from qualifying for any housing. She is currently living with my grandmothe with my sister in her care.
They lived with me thorough 2014 in which i provided complete financial support to both of them. During that time my mother made no attempts to find a job or pay me any of her portion of our expenses. She has definite signs of bipolar disorder and during the year she lived with me, she remained bed ridden due to severe depression for most of that duration. She also has a history of drug usage and her current husband abuses painkillers and has cobtibuted totheir heavy debt because of it. I have found paraphernalia in her belongings while she lived with me and took pictures for record. She also had random friends stop by at random parts of the day for only about 2 minutes maximum for what could potenially be for drugs since i know these friends also have history with drug usage.
We have different fathers but hers wants nothing to do with her. He has been in and out of court for neglecting child support payments for the last 8 years.
My mother refuses to seek help for her potenial disorder and shows no signs of improving.
My sister actually called me about 2 hours ago to tell me that my mom is taking her and all of the house pets to a friends house out of town indefinitely for a dispute she had with my grandmother. She had no intention of telling anyone that she was leaving or giving contact information to reach her by, hence why my sister had to sneak away to give me that information herself.
I want my mom to get help, but she has been like this and getting worse for years. She doesnt seem to attempt to parent my sister and my sister calls me if she needs anything. Is there a way i can get temporary custody over my sister until my mom gets help and stablizes her life out?

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She has also started drinking a lot lately. Alcoholism runs in the family and my grandmother also suffers from it

Not unless one of two things happens:

1. Mom agrees or
2. Your sister is removed from the home and placement with you is approved.

Just to note: Mom owes nobody an explanation of where she's relocating her child. She doesn't have to tell a soul.

Ok, slight update:
That increased drinking of hers that i mentioned has evolved into her having a two day binge, which subsequently resulted in her blacking out and smashing several items throughout my grandmother's house in an act of aggression. Also, my sister informed me that my mom is growing weed now. I went over for a visit, and saw that my sister wasn't exaggerating. Saw the new potted plant in my mom's bedroom and my mom confirmed it directly. My grandmother is also aware of this new hobby my mom has and seems more excited about than anything.
Is there anything I can do with this?

When people suspect abuse, they call CPS/equivalent.

Would this situation constitute as abuse? I could see it as child endangerment, but my mom has never been aggressive towards my sister, just everything around her. And i feel like my sister is being exposed to things like drug use that she shouldnt be.

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Quoting MH93

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Would this situation constitute as abuse? I could see it as child endangerment, but my mom has never been aggressive towards my sister, just everything around her. And i feel like my sister is being exposed to things like drug use that she shouldnt be.

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The point is you're just not going to get custody unless Mom agrees or the child is removed from the house by social services.

But didn't you just say that they'd left Grandma's house (which is where the questionable plant is)? Are they back there?