Mycroft was diagnosed with lung cancer at a young age, and was constantly - bedridden, in hospital, in rehab, etc. The truth was kept hidden from Sherlock, who simply began to think of his brother as an absentee figure in his live. When he went into remission, Sherlock was a little too old to really want to form a bond with his brother/need his brother as a central force in his life.

Now, all these years later, the cancer returns and it is Stage Four. Mycroft says nothing, but Sherlock is able to deduce the truth.