If you force it to use chain-of-thought: "Two fathers and two sons sum to how many people? Enumerate all the sets of solutions"

"Assuming the group consists only of “the two fathers and the two sons” (i.e., every person in the group is counted as a father and/or a son), the total number of distinct people can only be 3 or 4.

Reason: you are taking the union of a set of 2 fathers and a set of 2 sons. The union size is 2+2−overlap, so it is 4 if there’s no overlap and 3 if exactly one person is both a father and a son. (It cannot be 2 in any ordinary family tree.)"

Here it clearly states its assumption (finite set of people that excludes non-mentioned people, etc.)

https://chatgpt.com/share/698b39c9-2ad0-8003-8023-4fd6b00966...

Every father is a son to somebody...