How long should I keep a secret that my dad has brain tumor from the rest of my family?

When I last visited my family in September my dad had an emotional outburst to me while I went to his room to say sorry for getting annoyed w/ him earlier in the evening. He conveyed he had a brain tumor and not to worry that he would be around for a long time. He made me promise not to tell anyone else in the family. My mom had been going through a hard time w/ the declining health of her mother and he felt it wouldn’t be a good time to tell her. So I have known for months now and my dad refuses to speak to me about what exactly he has or when he will tell the rest of the family. My dad just retired and is only 63. I live far away so in part is why I think he felt he could divulge this news to me. I am just in a condulrum. I want to respect my fathers wishes but I don’t think its fair that my mother, and siblings haven’t been told. He recently was in town and stopped in very briefly to visit with me. On the way to his car I told him he needed to tell the family soon or he would be pushing me do something he may not like.
So my question is what the hell should I do? Should I wait until after the holidays to tell my dad, he has reached his limit and has a day to tell them or I will? Or should I respect his decision and let him handle it. I figure w/ the way things have gone this year, there will never be a “good time” to tell everyone. Any advice would be appreciated.

Lung Cancer