1) it has to be proved that death was due to medical negligence .
2) you have not mentioned whether you have the medical records or not .
3) please clarify .
if you have the medical records then consult another doctor . obtain his written opinion that there was negligence in treatment .
4) you can lodge complaint with medical council of india against hospital and doctor .
5) also lodge police complaint regarding negligence of hospital . police will send medical report to govt hospital to check whether there has been negligence or not . based on opinion of govt doctors case would be registered.
6) you will have to file consumer complaint for deficiency in service and for compensation in kolkata within whose jurisdiction medical negligence occurred