A said to B, “Your father’s wife’s son is my brother.” How is B related to A?

Correct — B. Shorten the chain from B's side.

Rule: your father's wife is your mother.

So "your father's wife's son" is a son of B's mother: B's brother, or B himself if B is male.

A calls that son "my brother", so A is also a child of B's parents. A and B are siblings, option (b).

Why the others are wrong

• (a)
  Cousin — A cousin is the child of B's uncle or aunt. A's brother is a son of B's own mother, so A is a child of B's parents, not of an uncle or aunt.
• (c)
  Uncle — An uncle stands one generation above B. A's brother is a son of B's mother, so A and B belong to the same generation.
• (d)
  Nephew — A nephew stands one generation below B. A, B and the son of B's mother all belong to one generation, so A cannot be below B.

Blood-relation chains get shorter when you replace each compound phrase with the single relation it names. "Your father's wife" is your mother, and "your mother's son" is your brother, or you if you are male.

After those two replacements the sentence reads: "Your brother (or you) is my brother." Taking everyone as full siblings, as the item does, two people who share a brother share parents: they are siblings.

Key facts

• Your father's wife is your mother.
• Your mother's son is your brother, or you if you are male.
• A and B share a brother, so the item makes them children of the same parents.

Common traps

• Looking for Brother among the options, when the stem leaves B's gender open and the key uses Sibling.
• Treating "father's" as a step up a generation and choosing Uncle, when "wife's son" brings the chain straight back to B's level.

No directly related past PYQ was found.