To determine how many sisters Sally has, we start by analyzing the information provided in the question:

"Sally (a girl) has 3 brothers. Each brother has 2 sisters. How many sisters does Sally have?"

---

Step 1: Understand the Family Composition

• Sally is a girl.
• She has 3 brothers, which means there are 3 male siblings in the family.
• The key phrase is: "Each brother has 2 sisters."

This tells us that from the perspective of each brother, there are 2 female siblings in the family.

---

Step 2: Interpret the Meaning of "Each Brother Has 2 Sisters"

• Since Sally is one of the girls in the family, and each brother has 2 sisters, that means there are 2 girls in total in the family.
• One of those girls is Sally, and the other is her sister.

So, the family consists of:

• 3 boys (Sally's brothers),
• 2 girls (Sally and her one sister).

---

Step 3: Answer the Question

The question is: "How many sisters does Sally have?"

• Sally is a girl, so she doesn't count herself as her own sister.
• There is one other girl in the family — her sister.

Thus, Sally has 1 sister.

---

Final Answer

$$ \boxed{1} $$