Hello and welcome to JustAnswer.
I'm sorry to hear about your situation and hope I can help.
Unfortunately, as an at-will employee, your sister can be terminated at any time for any reason with or without prior notice, even though she was on temporary disability at the time of termination.
This stems from the employment at-will doctrine, which states:
"An employment, having no specified term, may be terminated at the will of either party on notice to the other. Employment for a specified term means an employment for a period greater than one month."
However, if your sister were terminated due to her disability and would have been able to perform the essential functions of the job with a reasonable accommodation she would have a claim for discrimination in violation of the Fair Employment and Housing Act.
She can file a claim using the instructions available online here:http://dfeh.ca.gov/Complaints_eCompProc.htm
The Department of Fair Employment and Housing will investigate the complaint on her behalf and either file suit on her behalf or issue her a right to sue letter so she can pursue her claim against her employer through a private attorney.