bank denying to close account after my fathers death

My father has two accounts in same bank but different branch, in both he was the first holder, in one account I am the second joint holder and in the other savings account my sister is the second holder, now after his death I approached bank with my father death certificate to remove his name from our account and make me sole holder, bank is asking me to bring my sister to close the other account as well, I dont have any connection with her as she has no relationship with us, what to do