@Brd709 This has been an issue a long time. The game is programmed to go to the nearest sink. To use the metaphor "as the bird flies". So think 3 dimensional, the sink upstairs is technically closer. The up and down doesn't seen to count either for actual distance, actually. If you flattened all floors of the house out onto a 2 dimensional plane, the 'closest' sink is where they go first.
I have a build now that I downloaded from the gallery. A really nice large building. But the Sims will go through a maze of hallways and stairs to reach the sink in the 3rd floor bathroom after using the first floor bathroom because the sink on that floor is directly above the toilet on the first floor. The sink in the bathroom on the first floor is about 6 tiles away from the toilet.
I haven't tried it yet, but I'm guessing since the toilet on the 3rd floor is above the sink on the first floor, when they use the toilet upstairs, they'll make their way down to the sink on the first floor. :D
Crazy I know. Something I watch for when playtesting my own builds.