Explaining the sharia perspective on this type of issue, we would like to cite the following response of   Dr. Monzer Kahf, a scholar in Islamic Economics and Financial Expert , to a question of this nature. He said:

Of course, you must share all his money with other heirs in accordance with sharia inheritance distribution. A joint account is merely a convenient measure so that a jointer can withdraw the money from the account instead of waiting until all the necessary papers and procedure of property transfer be done. All the money in this account that belonged to your father at the moment of his death, along with all other properties owned by him at that moment must be distributed according to sharia to all heirs. In fact, all that became owned by all the heirs by the incidence of death.