What's wrong with making a collection called #$Brother?

Suppose Billy and Peter are brothers, and we want to tell Cyc this by asserting the following two facts:

(isa Billy Brother)
(isa Peter Brother)

Later, we learn of two other brothers, Jack and Bobby. We tell Cyc about them in the same way:

(isa Jack Brother)
(isa Bobby Brother)

Now, to ask Cyc for instances of #$Brother, we ASK:

(isa ?WHO Brother)

And we get the answer:

4 Answers     ?WHO 

-------------------
 
[Explain #1]  Billy 
[Explain #2]  Peter 
[Explain #3]  Jack 
[Explain #4]  Bobby 

So, Cyc thinks that they are all brothers, because there is no relation that distinguishes which of them are related to which others.