Normally as a beneficiary under a "revocable" trust you do not have any rights because the trustor can change the trust at any time. That is why it is revocable.
In your situation if your mother has dementia, in a legal sense, she cannot change her trust and she cannot enter into contracts.
What would be needed is a Petition in the Probate Court for Conservatorship. A Conservator, a person who is the closest living relative and a responsible person, would be appointed to manage your mother's affairs.
Then, perhaps, a Petition to Instruct the Trustee (who is probably your mother at this time) would be in order and maybe even a complicated legal procedure called a Petition for Substituted Judgment, at some point in time.
Again, if she was really diagnosed with senile dementia, then no one should be allowed to bend her will. Otherwise, someone could sell her the Brooklyn Bridge, right?
I'll agree with the previous response--you are probably going to need to have a conservator appointed in this case. Moreover, it seems like it would probably be in your best interest to immediately contact a probate attorney in your area who handles this sort of work. I would recommend moving on this now--because if you try to challenge it after the fact your sister could likely hire an attorney (at the expense of the trust) to defend the action and you may be caught up in a no-contest clause problem if one was included in the trust.
*DISCLAIMER* This answer is based on California law; the laws of other states may well lead to a different response or suggestion. This answer does not constitute legal advice nor does it form an attorney-client relationship between the questioner and responder. You should seek competent local legal counsel to address any specific legal concerns you have.
Was the Trust changed before or after the diagnosis? If after, get an estate attorney or litigator and they will have your sister removed and possibly surcharged for any misuse of Trust money. If the changes were made before diagnosis your attorney can still prove that Mom wasn't capable of making the changes and that your sister exercised undue influence over your mother.
Be careful while you are in the process of getting the estate put right that your mother is not neglected or harmed by the resident sister. This may be an appropriate time to get Adult Protective services involved.
This sounds like a situation where you should act quickly. HOWEVER, BEFORE YOU DO ANYTHING consult a licensed estate or litigation attorney in your area and go over the case with the attorney presenting all of the facts and supporting documents you have. This may help the attorney advise you with more precision, about how to protect your mother and yourself..