Posting here for traffic.
Should I tell my DH's family that I think he had an hereditary condition even though it hasn't been formally diagnosed?
They wouldn't have any health risks from this condition but could be carriers and may have passed the gene on to their children, who may also be carriers.
25% chance of 2 carriers having a child with the condition which would become evident quickly and is treatable - although it would need lifelong drug treatment.
YABU - say nothing. Why worry other people when you have no diagnosis.
YANBU - of course you should tell them what you suspect.
Just to clarify, DH has sadly died so is in no position to tell them himself.
The condition did not cause his death.