At one place you have written
"I have a FD in my name in a joint account with my father, step mother and grand mother"
At another place you have written "The FD solely carries my name and is not matured yet."
What is the correctly position? Is it that the FD receipt or the FD ledger account in the bank, bears name of all four i.e you, father, stepmother &grandmother? Or is it only in your name?
I shall take this question first legally and technically; then I shall take it from the moral or ethical side.
Legal & technical side:
Situation 1. FD ONLY in your name
If the FD is ONLY in your name, the bank will give the amount on maturity to you only. If it is only in your name you can even close it before maturity.
Situation2: The FD is in the name of all the four :
Then the bank will see what is the operation instruction-former or survivor, jointly etc. If it is former or survivor, the all the remaining three will get it jointly on requesting and signing jointly. They may even credit it to a joint account in all your three names. If there are no special instructions then the bank will give the amount to you three and any other living legal heirs of your father jointly on putting up a 'claim'. All the claimants have to submit proof of their claim right and identity proof. If the amount is large then there are some more procedures. First of all you have to submit the death certificate copy to the bank and get that noted in the FD records and any other accounts he is having in that bank.
If the FD has joint names , but still there is a nomination signed by all the joint holders, then nominee can claim the amount and bank will give the amount to nominee on identification with proof.However if the bank gets objection from any of the joint holders before giving the amount to nominee, they will stop that and freeze the money until things are cleared by all those involved or by a court order.
Insurance claim amount: Legally you need not share the amount with anyone as you are the nominee.
(However in all these case if anyone is taking up the matter in a case in court, then the same is subject to court order)
Ethical and moral side:
If the money is fully your father's earnings , then the proper way is to get the money of FD and insurance and divide the total among all the four equally; namely, yourself, your sister, your step mother and grandmother. Or you can come to a mutually consulted and considered understanding acceptable to all. If the amount is large it is better to put the understanding in writing in a stamp paper (agreement) with a common witness and all you four should sign and keep a copy each. You can keep the proof of having distributed the money as per understanding, along with the agreement.