What can I do to protect my mom from my sister who is successor trustee

You'll likely need to see an elder law attorney to see what you can do. You'll need proof that your sister is guilty of fraud before going to court. At any rate, a consulting session with an attorney would likely be a good idea.
Take care,
Carol

You need an attorney who practices in the county in which your mom lives.

To make the best use of your time with the attorney get copies of all legal items in advance: copy of the original trust document; if mom has in fact been sued a copy of the docket for the lawsuit(s) and a copy of whom was served, you can just go to the courthouse and get these for a nominal fee; copies of all other legal like mom's marriages, divorce, and spouse info (death certificate); and all banking info; and any old legal - like the will that the trust replaced. Also do a timeline on what you think did or did not happen that affected your mom's health care and financials. Providing this will cut down on the $ spent by the paralegals as well as give the attorney a realistic picture of what you feel the situation is and if there are grounds to investigate.

Fraud in an intimate or family situation is very hard to prove as often what you have is bad judgement rather than deliberate misrepresentation.