As to your question about whether there is a standard judicial council form that you may use for objecting to appointment of your sister as administrator, there may be one specific to Alameda County. I would recommend that you search the Court's website there in Oakland. Down here in Los Angeles County, we do not have such a form. Usually, we file a competing petition, and if we are objecting to the appoinment of another person's petition, we do that in the form of a pleading.
Alameda County does not have a form for objecting to the appointment of an administrator. As the other attorney said, you need to file your own petition to be appointed as administrator. You will also need to file an objection to your sister's appointment (which you will have to create yourself or hire a lawyer to assist you).
The information you obtain at this site is not, nor is it intended to be, legal advice. You should consult an attorney for individual advice regarding your own situation.