I would say try and stay strong for your Dad but also do not be afraid to cry about it. Thing is as awful as it sounds, sometimes the people who are closest to people suffering with illnesses like this get forgotten. People seem to forget to ask them how they are coping etc. Do not be afraid to voice your feelings....if you are feeling angry tell someone, upset tell someone etc etc. These are all normal feelings when dealing with a situation like this. Also do not be afraid to ask questions if you feel you do not understand some of the things that are happening.
Also if you feel you need some extra help go to your doctor and they may be able to put you in touch with some people who are outside the situation that will be able to help you. This is sometimes easier as you do not worry about upsetting the other people in your family.
I hope this helps you and please remember there are always people from outside any situation that will be able to help you and listen to you.
There are things that are not in our hands,this is one,you want ,desperately to help your dad,but you can't .
I'm sure,the doctors are doing all they can,to help your dad.
Stay strong, we all are praying for you and your family.
Talk to your dad,tell him all you would like to tell him,with serenity.
He is listening.I wish you peace.