I wasn't told my father was ill. What should I do?

My father is in a care home, he is still married to my mother although he has been living with another woman for quite a few years.
His partner is looking after everything financially and when I went to see my father in the nursing home this weekend I told one of the carers that he was complaining of pain in his arm she said that this was due to his illness bearing in mind I thought it was his diabetes she told me he was very ill and that the most they can do for him is make him comfortable I thought this strange at the time but didn't question anything further, I came home and my brother telephone me to tell me that he had been talking to my fathers partner and that my Dad has been diagnosed with Non Hodgkins lymphoma I am a upset and angry that I have not been told sooner, apparently they were going to give cancer treatment but he isn't well enough, I am very angry that I have not been told the same thing happened when he went into the nursing home I know there was talk about it but no one told me. What should I do?