You have a tough one by the tail here. We are not doctors but rather come from the school of experience. There are many layers to your issue as I see it:
1. Aging parents need to be able to retain some dignity and some independence. Your sister meant well but "parenting" your mother the way she did is more likely to have the opposite effect.
2. Your mother's symptoms and feelings are real, they are not all in her head. Depression is not an emotional disease, it is a physical disease that manifests itself in emotional ways. This makes it difficult for people who have depression to feel strong and confident. Your mother needs to believe and feel that she is a strong, competent and capable person in spite of her depression.
3. People react very differently to medications. I took Lexapro and had no ill effects at all and many others have experienced what your mom reports. It is real and Lexapro may not be right for her. If she is willing to stay with one doctor, it will have to be a doctor who makes her believe that she is valued and taken seriously. Try to find a doctor that specialises in senior citizens. Check with her local hospital to see if they have a senior care program. If she has someone that she believes understands, she will be more likely to listen.
4. I don't know everything about your mother's situation but it sounds like lonliness is part of the issue for her. Many older women are not used to doing things on their own and need some help and encouragement to rebuild their lives once their children are off on their own (no matter how old they are when that happens). See if you can find some local groups that are of interest to your mom. Does she quilt? There are usually many quilting circles where she could meet other women her own age. Does your local hospital have an adult volunteer program? Perhaps she could volunteer at the hospital. Does she like to read to kids, schools are always looking for mentors and adult reading tutors. Is there a senior center in her town? Everyone needs to feel useful and needed. Your mom is isolated and remaining that way will not help her.
I'm afraid that is the best I have to offer although I know others here will jump in too. Unfortunately you can not "make" her listen to advice or want to get help. She has to take charge of that part herself and want to get help. Your mom may get angry when people talk to her directly but if you do it with love and not in a parental tone, perhaps she will know that you are interested in helping. I know this is terribly difficult on you but walking away from her will not make this better either and I don't think you really want to walk away.
My last thought is that being a caretaker from near or far is very draining. Keep coming here for support while you work this out with your mom. You need to take care of you too.