The Indian Casinos are suppose to use a Workers' Compensation system that mirrors the CA Workers' Compensation system. Instead of using the Workers Compensation Appeals Board, workers who are injured at the Indian Casinos need to appeal their denial to the Intertribal Council for the region. Not all tribes are members in the San Diego area, but most are.
This answer is not intended to be legal advice and an attorney licensed in your jurisdiction should be consulted on this answer and for any other questions that you may have. Further, by answering this question, an attorney-client relationship has not been created.
Indian casinos, because of their independent nation status, do not fall under California workers' compensation laws. Your sister-in law is going to have to find someone in San Diego who does workers' comp that also does the Indian casino cases.