Before the US was so identity heavy[1], it was not an uncommon phenomena for a family to raise a child as their own[2] and the child not be told. That crazy uncle or great aunt might find out and might not.

In more modern times, questions generally get asked as a result of some blood tests or organ compatibility tests.

1) birth certificate for everything, although I remember an old story that the birth certificate might have been written by a sympathetic official

2) sadly, some stigmas were pretty great

The older brother was also requesting most of their parents heritage and the others weren't frilled by that. It seems their mother first realized the possibility but never bothered to check, and the inheritance problem was a good timing for the siblings to go down the rabbit's hole.

Now, should a lack of biological link count if you've been raised the same as the other natural kids is up to debate, but at least the brothers think that this will push further their position, outside of all the more healthy 'we met at last' feelings.