What is the ruling on concealing from my family that I have the same chronic illness as my wife, so as not to sadden them?

If informing your family of your illness would distress them, then do not inform them unless concealing it would cause harm. This world is a place of trial, and the reward for patience in the face of tribulation is immense. Illness expiates sins, as stated in the Hadith: "No Muslim is afflicted by illness, or pain, or sorrow, or even a concern that preoccupies him, but Allah, the Exalted, expiates some of his sins thereby." And know that illness is by Allah's decree, as He, the Almighty, said: "No disaster strikes upon the earth or among yourselves except that it is in a register before We bring it into being - indeed that, for Allah, is easy." Contagion is a means through which Allah implements His decree. The Prophet, peace and blessings be upon him, said: "There is no 'Adwa (contagion) and no Tiara (bad omen)." So, beware of hinting to your wife that she is the cause of your illness, for that is a reproach and ill manners regarding Allah's decrees.