Let's analyze the problem step by step:

• Sally has 3 brothers. So, including Sally, there are at least 4 children: Sally (a girl) and 3 boys.
• Each brother has 2 sisters. Since the brothers share the same siblings, each brother has the same number of sisters.

Now, from the perspective of one brother:

• The sisters are the female siblings. Since Sally is one sister, and each brother has 2 sisters, there must be another sister besides Sally.

Therefore, the family has:

• 2 girls (Sally and one more sister)
• 3 boys (the brothers)

So, Sally has 1 sister (the other girl) and 3 brothers.

Thus, Sally has 1 sister.

Final Answer: [ \boxed{1} ]