Logic Puzzle

Here's how to solve this:

1. Focus on the brothers' perspective: Each of Sally's 3 brothers has 2 sisters.
2. Identify the sisters: Since Sally is a girl, she is one of those sisters. The other sister must be another girl in the family.
3. Determine the total number of girls: If each brother has 2 sisters, there are only 2 girls in the family (Sally and one other sister).
4. Count Sally's sisters: If there are 2 girls in total, and Sally is one of them, then Sally has 1 sister.

Answer: Sally has 1 sister.