Unless you know the name of the bank and insurance company, you will need to go to court to obtain this information if your stepdad won't cooperate. If there is a probate proceeding already opened for your mother's estate, you should be able to find out this information in the probate proceeding. HIre a local lawyer to do this.
If you know the name of the life insurance company, call them directly, and if you are the beneficiary, they will tell you. If not, they won't give you any information.