Your side comment about saunas got me searching. It's quite amazing (though based on small populations) - I'll be giving this a try, thanks for pointing it out!
Sally has 2 sisters. Each brother counts Sally and two other girls as sisters, which means there are three girls in total. Since one of these girls is Sally herself, that leaves two others as her sisters.
When I said 'nope', it corrected itself and gave the right answer.
https://www.mayoclinicproceedings.org/article/S0025-6196(18)...