[ijustlovemath]

Kagi's expert mode managed to handle this (rest directly from the chat):

Based on the search results, we can break this down step-by-step:

    Sally has 3 brothers.
    Each of Sally's brothers has 2 sisters.
    Since all the brothers share the same 2 sisters, we are hearing about the same 2 sisters each time, not additional sisters.
    We know Sally is one of the 2 sisters that each brother has.
    Therefore, there must be one other sister that the brothers have besides Sally.

In conclusion, Sally has 1 sister. The key thing to understand here is that when it says "each brother has 2 sisters", it is referring to Sally and one other sister that all the brothers share.