Legal Question in Administrative Law in California

If 'passed' means dead or even 'legally incompetent', you can't.

If alive and legally competent, she would have to execute a properly prepared Power of Attorney.

If she is 'legally incompetent' needing someone to manage her estate or medicals, you'd need to apply for a Conservatorship or Guardianship through the Probate Court. If so, consult with local probate counsel for help.