to employ an impossible scenario (two of her. one walking up and one walking down. would they meet? no because it's impossible for there to be two of her existing in two different places at the same time.) which is kind of the point of thinking "outside the box" of reality. Because the answer would be too obvious if the question were worded:

At sunrise a nun leaves her monastery at the base of a mountain and walks on a narrow path to a small shrine atop the mountain. Also at sunrise her twin sister leaves the shrine, and descends down to the monastery following the exact same path.

Estimate the probability the two sisters passed some point on the path at the same time.