Oh, those long lost family members
Neal went into witness protection at age three, along with his mother and Ellen. He did not grow up with an extended family around him. If he has any memories of family other than his mother, those memories would be very vague. So, Neal's probably not going to be reminiscing about uncles or aunts.
Furthermore, the chances of Neal having a sibling are extremely small. He promised to tell Peter "everything," and explained WitSec, told the Peter everything he knew about his father, etc. The idea that he left out all mention of a sibling, a relatively innocuous fact, is unlikely. Ellen also makes no reference to a brother or sister.
And even if he did have a sibling, they're unlikely to just walk into the FBI building to say hello. Finding Neal would prove to be highly difficult, given his name change, aliases, and constant moving for most of his adult life.
Obviously, it's a different story if the fic was written before the WitSec revelation. But stuff written afterward has no excuse.
(There are also AU's of course, but it just doesn't fit in with canon.)