I'm none of the three people in this story but I am close to them and would like to get a reading on what people think is the right thing to do.
3 siblings - A, B, and C. All three live a long way apart from each other. For a number of years, B and C have not been on speaking terms. A and B are fairly close, A and C are not close but do vaguely keep in touch.
C tells A they have a terminal diagnosis. Don't know how long they have left, but it should become clearer after some further tests very soon. C tells A they don't want B to know. A is concerned that when B does eventually find out, it will seriously impact their relationship if they find out A knew and didn't tell them.
What is the right thing to do?
YABU - it's C's news, so their wishes should be respected. Don't tell B.
YANBU - A should tell B and give them the chance to patch things up while it's still possible.