So sorry to hear about your dad, Stephanie. It is difficult enough without his request for "secrecy."
As you know, without treatment your dad would have no hope of recovery. Many patients undergo a radical cystectomy and live very happy and complete lives for many years. We have members on this Forum who have recovered from surgery/treatment and drive race cars, climb mountains, and scuba dive. A LOT of the success in recover has to do with the mind-set of the patient!
With respect to the request that you not share his diagnosis with your sisters, my opinion would be that if he is of sound mind his request needs to be honored. Of course you should ask him how he would feel if someone close to him did not share a similar diagnosis with him!! I know of some really sad cases where someone did not tell their family member because "they didn't want them to worry" and, from then on, said family member lost all trust in their loved one.
Please ask any questions you may have as you go along. There are many here with experience who can provide answers for you. For a start, and as a review of the facts about many aspects of bladder cancer, I recommend that you go to our Home Page
bladdercancersupport.org/index.php
and use the pull-down menus in the blue box on the left. AND ask us questions!!
Best of luck to you and your dad
Sara Anne