How do I deal with my controlling sister who thinks she knows "what's best" for mom?

Which one of you is the caregiver for your parent? If it is you, I would let your sister know that her comments are not helpful or productive and to keep her opinions to herself. Whenever she starts in on her negative rants, change the subject or have a convenient excuse for leaving the conversation. Stay calm and be firm.
If your sister is the caregiver, she may be overwhelmed with the responsibility and wainting for you to pitch in.
Cannot tell which is the case from your post.

I say if your sister thinks she knows what's best for your mom, let her go ahead and do exactly what she thinks she wants to do. Surrender the control to her and see just where she lands. Go over as a guest, and visit mom as her daughter, not her care-taker, (granted you are not).

Watch and see how overwhelmed she gets and how soon she'll be calling on you for help. I'll bet it wouldn't take too long!

I have the same problem with my sister. I am sole caregiver for my mother. My sister lives in New Jersey and I live in CA. She is constantly telling me what and how to do things. I used to find her input helpful, now it is just annoying. So, after asking her to only offer suggestions to help mom that she could do from New Jersey, I simply stopped having any conversations about mom with her. I told her if she wanted to know what was going on with mom to call her directly. I am doing the best I can and need help, not criticism. I told her to call me when she was ready with a positive attitude. In other words, don't let your sister's issues cause you problems. Ignore them and her if necessary so you can function productively and happily. I hope this helps.

I have a similar situation. My sister lives 25 minutes away, but does very little to help care for Mom. Mom is in assisted living, close to my home. She has severe memory problems and needs direction to dress, brush her teeth, etc. When we are out of town, my sister is calling me with complaints about the care she gets, and that she is "all alone" to care for Mom". I get e-mails telling me if the trash isn't emptied or the TV is not working properly. We tangle frequently because I keep telling her to deal with these issues instead of telling me to deal with them. I have not found a way to avoid this conflict. We have a brother who is 45 minutes away, but we never see him visit even. What a group. In the meantime, I am going over to Mom's every morning and every evening. I need help, not supervision!!

I manage an online caregivers group, and this scenerio has come up amonst others as well--The best advice that can be given is to let your sister take over, and she will then see all that is involved. Like many of the caregivers in this forum, we do need help and advice from time to time---bur NOT supervision. If your Mom is of sound mind aske her what she thinks-otherwise go with your heart. Be sure you have POA for both legal matters as well as health issues...as it will make life easier...with decision making.
Good luck on your caregiving journey, and do what works BEST for you as well as your MOm.
Hap

This is a great resource. Each response was helpful and compassionate. Thank you. I will try some of the suggestions in regard to my relationship with my sister.