Im starting to see Sachi is more mature than Erika which verifies Hotel King Dads claim of Sachi preserving everything if she and Nagi marries.
My theory is that Erika keeps living in an illusion of being a kid with an "imaginary" older brother since she was raised alone in actual reality. Since Erika has not developed mentally, Sachi surpasses Erika in the older sister aspect. THE IRONY.