I indeed found funny the idea of them discovering the 'tracks' of another uniform stealer (a bound-and-gagged guard in her undies). Given how many people steal uniforms in these stories, the characters are bound to come across someone who was mugged by someone else from time to time!
The two sisters were a call-back to the previous little adventure of Selena and Lucina, in which they also came across two guards who were also sisters. These guards chose the smartest way-out by agreeing to surrender. A nerve-pinch is a lot more painless than a beating!