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
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[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.