I'm really after your opinions on whether IABU, I really don't think I am, exH thinks its "none of my business".
Slight back story, exH has never "parented" our children, all decisions have always been mine, we divorced four years ago, he moved back to his parents, DC and I lived on our own, I have since met someone else and we have gone on to have another child and all good. I do not have a good relationship with exH, he sees the DC (his choice) for a few hours every Sunday. He told me a couple of weeks ago, he is happy being a part-time dad as it gives his time with his girlfriend and her children 
Anyway, 8 years ago, exH dear brother (who I thought the world of) was diagnosed with MND and sadly, he lost his battle six months later. The most horrendous and debilitating condition. ExH (one of a twin) has now announced to me that his twin brother has also now been diagnosed with the same condition, it has now been deemed hereditary. I am very shocked (and terrified) for my own children, rightly so? He asked me not to tell the DC (DS is 12, DD 8) or anyone else. I agreed, I need to process the information and assumed exH was being tested, as he said he was. He then decided, he won't have the test, which I feel slightly pissed about. Although I understand he is probably terrified, again rightly so, I'm not sure at this stage whether I want to know whether my DC also carry the gene - does this make sense? So we agreed, that until we had time to process this diagnosis, the DC would not be told.
Until, of course, yesterday. He decided to tell them, without any word of warning or discussion with me. He has taken that decision upon himself and I am so angry. He may be their dad, but he has chosen to have very limited contact with them, he takes no interest in their education, hobbies, nothing. And yet, he feels he is justified in telling them this major, possibly life-changing information, and its none of my business??
Not only that, since he has taken it on himself to tell our DC, he should also be telling the mother of his eldest son, and also the former wife of his eldest brother, whom has DC with before he passed away.
Sorry, long-winded I know.......
So, AIBU to be fuming that he took that decision away from me? And AIBU that I'm terrified for my DC?? 

So many emotions right now.
Thanks for reading.