Legal Question in Elder Law in California

You could take another crack at replacing your sister as conservator, assuming she was appointed, but there would be some complicating factors. First, you may have signed a binding settlement agreement that restricts your rights in this matter. Second, as a rule of thumb, in the law you usually get only one bite of the apple. The judge is likely to want to see evidence of new wrongdoings, rather than the dispute that was already subject to litigation. While the legal doctrine of "res judicata" probably would not prevent you from trying again, the judge's inclination would be too look at new evidence rather than old.

Finally, you almost certainly cannot hire the mediator to represent you. The mediator essentially represented both you and your sister in a position of neutrality to attempt to bring the two of you to a settlement. That should put the mediator in an irreconcilable conflict of interest in which he or she would be prohibited from representing you against your sister, or visa versa.