Sally has 3 brothers, meaning there are 3 boys in the family. Each brother has 2 sisters, meaning there are 2 girls in the family. Since Sally is one of the girls, there must be one other girl. Therefore, Sally has 1 sister.

This can be verified by considering the family structure:

• Total children: 2 girls (Sally and one sister) and 3 boys.
• From Sally's perspective: She has 3 brothers and 1 sister.
• From each brother's perspective: They have 2 sisters (Sally and the other girl) and 2 brothers (the other two boys, as they do not count themselves when referring to brothers).

If there were only 1 girl (Sally), each brother would have only 1 sister, contradicting the statement that each has 2 sisters. If there were 3 girls, each brother would have 3 sisters, also contradicting the statement. Thus, exactly 2 girls is the only consistent scenario.

Therefore, Sally has 1 sister.

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