As before stated, your best chance for a challenge is based on grounds of undue influence or incapacity. As to the incapacity challenge, the mere fact that your mother was taking medication for dementia is informative but may not be persuasive enough. You may want to find out if there are any physician opinions, or diagnosis that could be included in a statement to the court. Unfortunately, POA's are the most common tool for elder abuse and cases like this happen fairly regularly. If you feel undue influence was exercised, an experienced local attorney should be able to help you on this case. If you would like a recommendation for an attorney in your area, feel free to contact me.
It is not clear if the trust is revocable or non-revocable. Who is the current trustee your mother or your sister? You can certainly challenge the amendments to the trust made after you claim that your mother became incompetent. I suggest you consult with a probate/trust attorney.
DISCLAIMER: The response herein is not legal advice and does not create an attorney/client relationship or any right of confidentiality between you and the responding attorney. These responses are intended only to provide general information about perceived legal issues within the question. Each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer is not a substitute for competent legal advice from a licensed attorney that practices in the subject area in your jurisdiction and who is familiar with your specific facts and all of the circumstances.
The most common reasons for contesting a will or trust are incompetence or undue influence.
If this can be proven-you have a great chance to contest the changes.
The answer given does not imply that an attorney-client relationship has been established and your best course of action is to have legal representation in this matter.