Plus mean i get the point the author is putting, the little sister's guard is down coz she know the girl she is following kinda and isnt the point of their infiltration to find out whats going on? So make sense she'd follow along regardless of the situation for more information which was basically spoon fed to her by the queen. And since mc has the 3 strongest spirits i mean, u kinda would take a bit of risk. It will come down to what happens after little sister followed in to that room to know for sure how dumb little sister is