We ponder about whether or not a doctor should tell a patient they have a terminal illness if the family requests otherwise. I would say that it is up to the patient to decide whether they want to be fully informed or not, not their family.
The family do not really know how the patient is feeling, even if they belong to a very close knit, loving and fair family. Nobody can know how that person feels, even someone who has had the same illness. They probably feel as though they are out of control and long to have control back, so one reason they should decide this is so that at least they have control over this one big issue.
But you also have to question why the family do not wish them to know. It could be for all sorts of reasons, some of them thoughtful and caring, others maybe a bit suspicious. Supposing the family have never got on with the patient and do not really like them or care about them. The fact that they are related is no guarantee that they want what is best for the patient. Quite often our friends are closer to us than our family, who we may not have seen for years and are virtual strangers to us. And surely if there are four members of the family it is unlikely that all four will agree. You may have two who wish the patient to be kept in the dark about their condition and two who do not mind or who are quite happy or wanting the patient to know. The doctor is there to treat the patient and doctors are busy people. They should not have to waste time on or be dragged into family debates too. With the best will in the World the family might decide that it is in the best interests of the patient not to know, and get it totally wrong.
Whether or not the patient is religious should also have a bearing on the decision. A christian might welcome the news or not be too unhappy about it, and see it as a chance to make their peace with God, meet God and be with him, and put their affairs in order before they pass over to heaven. Anyone who truly believes in the after live, a spiritualist for example, would not see death as the end of life with no continuation. Someone who is in a lot of pain might see it as a relief that they will die, and look forward to having an existence without the constant pain.
Each patient has the right to make sure they have made a will, or have the chance to change it. If a family are allowed to decide they are not informed that they have a terminal illness then perhaps they would not put their affairs in order as they should do. This might be advantageous to one or more members of the family, but is unfair on the patient.
Of course there has to be a hard and fast rule about this where we generalise. We can hardly tell doctors that they must tell this type of patient but not that type and then expect him to spend his valuable time trying to work out which type this patient is, maybe getting it wrong. So if one has to decide a definite yes or no I would say the patient has a right to know, unless they have made it clear they would rather not know.