I would recommend that you first talk to your stepsister and advise her of your concerns and find a way to work together for the benefit of your stepmom.
If this does not work, then there are various legal remedies you may want to consider by first consulting with an attorney:
•A motion for a TRO and preliminary injunction;
•A Probate Code 850 petition to decide ownership of assets that your sister transferred;
•A petition to revoke your stepsister's as agent in your stepmom's power of attorney & medical directive;
•A petition under Prob C §§4541 to evaluate your stepsister's actions & to compel an accounting.
An attorney will assess your situation and decide the best course of action for your stepmom.