A man, pointing to a woman sitting in a park, tells his friend, "She is the…

Concept: In a blood-relation puzzle, the speaker is a grandson (not a son) of his own grandmother, so "my grandmother's only son" can never mean the speaker himself. Because a person typically has two grandmothers — paternal (father's mother) and maternal (mother's mother) — and the puzzle does not say which one is meant, "grandmother's only son" can map to two different relatives depending on which side is intended.

Application:

1. Paternal reading: if "my grandmother" is the man's father's mother, her only son is the man's father (assuming the father has no brothers). The daughter of the man's father is the man's sister.

2. Maternal reading: if "my grandmother" is the man's mother's mother, her only son is the man's mother's brother, i.e. the man's maternal uncle. The daughter of that uncle is the man's cousin. Since the statement gives no information to rule out either grandmother, both readings remain valid at once.

Cross-check: Option "Daughter" would require the man himself to be his grandmother's son, which is impossible since he is her grandson. "Sister" alone and "Cousin" alone each satisfy only one of the two equally valid readings, not both. Only "Either Sister or Cousin" holds under every valid reading of the statement, so it is the complete answer.