P and Q are brothers. R and S are sisters. The son of P is brother of S. Q is…

Concept

A blood-relation chain is solved by fixing generations one link at a time. Saying that one person is the brother or sister of another places both in the same generation as children of the same parents, so a parent that is known for one of them can be carried across that sibling link to the other. Once a person's parent is fixed, the parent's own brother stands one generation above that person, and the term for that position is uncle.

Applying it to this chain

1. The son of P is the brother of S, so P's son and S are children of the same parents; this makes S a daughter of P.

2. R and S are sisters, so R has the same parents as S; therefore R is also a daughter of P, and P is the father of R.

3. P and Q are brothers, so Q is the brother of P, who has just been fixed as the father of R.

4. The brother of a person's father is that person's uncle, so Q is the uncle of R.

Cross-check against the other relations

• Son would need Q to be a child of R, but Q sits in P's generation, one above R.

• Brother would need Q and R to share parents, whereas R's parent is P and Q is P's sibling.

• Father is the role already fixed for P through the sibling link with P's son.

• Uncle is the term that means the brother of a person's father, which is exactly Q's position with respect to R.