My sister and I share POA for my mom (84) who suffers from dementia and bi-polar disorder. My sister recently tested positive for cocaine at her job. She has a history of alcohol abuse, and in 2015 she was hospitalized for an overdose and was on life support. My mom lives with her. I am worried about my mom being in her care. There has been no abuse or neglect, but my sister is insisting it is her right to continue drinking.
Meanwhile, is there another place for your mom to live besides with your sister? Your sister's life is unmanageable. I imagine she can barely care for herself much less care for a mentally ill elderly mother with bipolar and dementia.
I suggest consulting an attorney about the co-POA.
Is your sister financially dependent upon your mom? If she is you could be in for a rough time if you try to remove your mom from your sister's care but that's where a lawyer's advice comes in handy.
As someone suggested, sometimes we have to wait for our parent to have a health crisis before we can change anything. My family had different dynamics, there was no addiction or anything like that, but I waited until my mom had to go to the ER before I sounded an alarm. I took the nurse aside privately and told her about my parent's situation and asked her if she could speak with the Dr. and see that my mom was admitted so we could make some necessary changes in my parent's living situation. The Dr. agreed and we were made aware of several options we weren't aware of. We had a social worker who worked with my brother and I and my dad was on a need-to-know basis which is what we needed. My point is that between the nurses and the Dr. and the social worker we got things figured out. We didn't end up having to put any plan into motion as my mom died of natural causes while still in the hospital but I was and am grateful for the education. I actually created the "emergency" to get my mom to the ER. She had been falling repeatedly and my dad would call me to come and pick her up and put her back in bed. That last time she fell I feigned illness and said I was unable to get my mom up off the floor and that we'd have to call 911. That didn't go over well but I insisted and that got my mom to the hospital. I never regretted the fib. It had to be done.
Although you said there's no abuse or neglect I don't think your mom is safe with your sister anymore than a child would be safe living there. An alcoholic can't adequately care for another person, especially when that person has the special needs your mom does. You're right to be concerned.
If you can't find a Dr. willing to declare your mom unable to care for herself, and you can't convince your mom to withdraw the POA from your sister, then, sadly, you might have to wait until her mental and/or physical health is poor enough that a Dr. will declare her unable to live on her own.
I know that will be hard on you, but realize you're not alone. It's a serious problem for many kids dealing with their elderly parents these days. The law is there to protect our elders from being forced to give up control of their lives by someone else until someone in authority can prove that their mental or physical state is such that they are no longer able to wield that control. Sadly, it often takes an accident, or serious health issue, before a Dr. (or judge) will go on record that the person in question does need help with their lives, but that is reality. You can then just try to do what you can to help keep mom safe.
If you can not do anything now, it would be best to not try to get on the bad side of your sister, so keep quite about your ultimate goal. You are going to want a open communication with her so you'll know what's up with your mom on a regular basis since she lives there. Remember, you're not your sister's judge, and you can't change her...so work closely with your sister, biding your time, until you find your chance to step in and take over.
Good Luck and my prayers are with you.