In my opinion you need to decide whether you are going to talk to your FIL as family or as an employer. I'm currently going through issues with a connective tissue autoimmune disease (no diagnosis yet) and my employers have flat out said "We don't want to know and we can't know". I send in paper work because I miss a lot of work due to fevers/pain/fatigue but otherwise, no one knows about my condition.
I'm very secretive about it as well. My close family knows, and my good friend, I complain to some people about it for the simple fact that I need to vent myself, but like Just M said, I get worried about people not believing me or taking me seriously. I'm only 21 - how can I be going through all this pain? How can I have stiff joints and all this fatigue? I'm sure a lot of people think I'm lazy. I Thought I was lazy for a long time until I realized there was something wrong with me. It's hard to keep up "normal" when you aren't normal, but in my house we do what we can and I have nights off.
You have every right to be "Selfish" (which you are NOT being) and to not tell people. They honestly don't need to know. They can know when you need/want them to know. If it matters to them or they are your emergancy support, you need them to know, but otherwise it's none of their business. It can be embarressing to share it...