How to get a release to contact my mom's doctor but my sister will not give me information about her present condition?

You would have to provide the MD with those legal documents. Read them carefully to be sure you know you have proxy authority.

What I would do if you know the doctor's name is call that doctors office and tell them who you are and that you have authority as a co-trustee. Arrange to send them a copy of any papers you have, this is the route I would take and I would start there. If this fails, they should be able to tell you what to do next

Send a copy of your legal documents to your mother's doctor, wait, then call him/her and ask questions. Siblings are a pain in the neck most times!

Lots of times the doctor gets frustrated when more than one family member wants to be contacted for information, especially repeatedly. Why can't your Mom tell you who her doctor is? If she is that incapaciated, and you have a durable power of attorney for healthcare, then you get that to the doctor and you have a right to talk with him. Or you can go to the doctor's office with Mom and get permission from Mom to always talk with her doctors, even without a DPOA if Mom is competent enough. Trustee or executor status does not give you authority to get info from the doctor. You must also work on a better communication with your sister for matters taking care of your Mom, somehow.
Thats not fun, and its not all your fault, but you two have to look at this as well.

You might have to schedule a trip to see mom. You could solve your problems best. Check for cheap flights.

If you are named as co trustee and also on her health directive forms, then you do have a legal right to communicate with her doctors. If your sister is refusing to give you the doctor's contact info, then I would call the law firm that drew up those papers and explain the problem you are having with your sister. The lawyer should be able to contact your sister and tell her she must get your requested information to you. IF you have phone contact with your Mom, she might be able to tell you, unless she has dementia so bad she cannot remember. Is your Mom living with your sister or in a facility? If in a facility and you have this paperwork, you could fax copies to the facility and ask them to provide you with the doctor's contact information. I certainly hope that YOU have your own copies of all this legal paperwork, if you are a co trustee etc.too....and if you do not, that is the first thing I would be getting from the law office. I do wonder WHY your sister is not responding to your request??

I would take a trip to see mom and help with her caregiving, get involved, take her to her doctors appointments, and when there, talk to her doctors, help your sister. That's usually how it's done, involvement in the care giving process to some extent.

What's your sister's problem with answering your questions?

Once again, being a trustee does not confer any right to medical information. I have heard this idea too many times. Also, people will say that because they are executor they have this right. If you are looking for a legal right, that would be the DPOA for Healthcare, or even a general POA will usually be honored by the medical information sharing laws. Recognized relationships and being along with Mom for visits will always get a person the very best results on learning things from the doctor.

You must call mom's doctor and get your name added to the "allowed contacts." There does exist such a list.

you being named as co in healthcare directive allows both of you to access at same time. If mother in Facility. Send copy fax with coversheet stating legal document. HIPPA Does apply with mothers name .
This should go to admissions to be scanned into her chart profile.
I just went through this again.
They shreded his file when they did illegal discharge.

The OP can't approach the doctor to present her credentials because she doesn't know who her mother's doctor is, and apparently her sister will not share this information. So it seems that communication has broken down quite badly. That is why I was wondering if it were for reasons that might be put right.

But since the OP hasn't been back to comment...

I'm sort of confused how you can be "caring for mom" but you live 1500 miles away and have no idea the name of mom's doctor. I don't mean to be mean, but if you were really caring for mom all along, wouldn't you already know all this? So now, to get up to date information about your mom, you are encountering some type of resistance from your sister, is that correct? And she has been the one who has been doing all the local care and connections with doctors? So it sounds like sister has everything set up the way that works for her and your mom, and now you would like to be part of that system too. This will take time, first of all, and you might encounter quite a bit of resistance not just from sister, but also as someone else already noted---the doctors. Most medical clinics are SO busy, they do not want to have to deal with yet one more family member. So you do need to nicely send them the co-medical poa documents, and ask them to keep you informed. Of course first of all--you need sister to tell you who the doctors are! To do that: send a written letter nicely requesting the names of all doctors, pharmacies, etc, and send to sister Registered Mail, Return REceipt requested. Put a reasonable time frame on her response to you. If sister does not comply, then you could contact the attorney who drew up the poa documents for your mom, and ask the attorney to get this information for you. Be prepared that this will cost you some money.
After you have the access that you dearly want, be sure to schedule in-person visits with your mom, not just phone calls! There is SO MUCH that cannot be understood over the phone. I suggest you set aside at least a week, once a year, or 3 or 4 weekend visits spread over the course of the year. Also set aside an emergency transportation fund in your personal savings, so that if need be, you can fly out to be at your mom's bedside if she is urgently ill.
If you are not prepared to spend ACTUAL IN PERSON time with your mom, then please do not pursue too much involvement, unless your sister is clearly doing an inadequate job, it is much more efficient all around to just have the local relative handle the communications with doctors. Remember these doctors and facilities are so very busy, and they really do not like having to repeat the same info twice.
So go ahead, and be involved---but don't be lukewarm about it! Embrace the caregiving, in a REAL, substantive way. If you cannot do this, then step aside.