[jaykru]

Gemini advanced:

This is a bit of a trick question! Sally has one sister. Here's why:

    Sally and her brothers share the same sisters.
    Since Sally is a girl, she is one of the two sisters that each of her brothers has.
    Therefore, there is only one other sister besides Sally.

---- Gemini original:

There are two ways to think about this question:

    Including Sally: If we consider Sally herself as one of her own sisters, then she would have 1 sister (herself) + 0 sisters (additional sisters besides herself) = 1 sister.
    Excluding Sally: If we only consider sisters other than Sally herself, then each of her brothers has 2 sisters, but they all share the same two sisters. So, there are 0 sisters (additional sisters besides herself) = 0 sisters.

Therefore, the answer depends on whether you include Sally as one of her own sisters. If you do, then Sally has 1 sister. If you don't, then Sally has 0 sisters. ----

ChatGPT 4 (YMMV, I think I may have a custom prompt set):

Sally has 2 sisters. Each brother having 2 sisters just confirms Sally plus one more sister. ---

Wasn't expecting Gemini Advanced to clinch this one.