Opening Round Draw

A chess club runs a single-elimination knockout with 16 players. The bracket is drawn completely at random: every player is assigned uniformly at random to one of the 16 bracket slots, and the slots are paired up (1 with 2, 3 with 4, and so on) into the 8 first-round matches.

Two of the players are the reigning champion and her younger sister. What is the probability that they are drawn to play each other in the very first round? Give the answer as an exact fraction.