Unfortunately your only option is to pursue conservatorship proceedings. The conservator will be able to get the trust and can look into the propriety of what you sister is doing. The court can also freeze or void any powers or documents your sister created or got your mom to sign. You should do this sooner rather than later.
Look for a Elder Law attorney with trial and litigation experience. You need someone who is not afraid of confrontation.
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I am sorry for your circumstances. Unfortunately, in your case, you are not entitled to copies of your mother's estate planning documents while she is still alive. With that said, you can retain an experienced probate attorney and petition a probate court to become your mother's fiduciary. As such, you will manage her finances and make her health care decisions. Of course, your sister is likely to oppose you, which is why having the best legal counsel you can afford is going to be vitally important. Good luck to you.
This information is presented as a public service. It should not be construed to be formal legal advice nor considered to be the formation of a lawyer/client relationship. I am licensed in Connecticut and New York and my answers are based upon the law in those jurisdictions. My answer to any specific question would likely be different if I were to review a client's file and have the opportunity to interview the client. Accordingly, I strongly urge you to retain an attorney in your jurisdiction with respect to any legal matter.