Final answer:
The best definition of 'furrow' in the context provided is 'wrinkles made in a face or skin', describing physical facial changes due to emotions.
Step-by-step explanation:
The word furrow as it is used in the passage most accurately defines wrinkles made in a face or skin. This interpretation is clear from the context given, which describes a person with furrowed brows, commonly understood as a facial expression where the eyebrows are brought together, causing wrinkles on the forehead due to concentration, worry, or anger. n this context, the author is describing how the older brother's brows were furrowed, which means they were creased or wrinkled in a way that resembled a narrow groove in the skin. Hence, it depicts a physical change in the face, often associated with an emotional state.