I would hire an attorney to discuss guardianship and abuse issues and determine a plan that would be best for mother.
Every story has two sides and depending on the level of dementia-it might be that mother can not live by herself.
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.
Consult with an attorney in your area familiar with trust administration, conservatorships and probate litigation. The attorney can advise you as to what to do to protect your mother or her assets and your rights if any.
To find an attorney, you may want to use Avvo's "Find a Lawyer" above or contract the San Francisco Bar Association or the California State Bar Association for a referral. Often these attorneys will have a free initial consultation.
I am licensed to practice law only in the state of California. The answer provided above is for general information only, is not intended and should not be taken as specific legal advice and does not create an attorney client relationship with the party making the inquiry. The best way to contact me is by email at firstname.lastname@example.org.
Please clarify, your Mom is alive, unfortunately with dementia and has a living trust. Your sister is the back up trustee for you Mom as she is unable to administer the trust?
Your question is not clear if your Mom is still in the home or not and not quite sure about the reverse mortgage.
Regardless, If you feel your Mom is being abused or taken advantage of, immediately contact Adult Protective Services -- I attached the link below.
Also contact Legal Aid for assistance as well -- see link below.