Just a theory ,but could Erika be suffering from an illusionary mental illness illusion of having an older brother and Nagi's dad already knows this thats why he offered Nagi the other alternative to continue the bloodline through his younger non blood related sister of marriage?
They key to this will be that recent Beach House, height wall carving and that suspicious game saved, is my assumption.
So Erika may not have all that screws in place and for Nagi's own safety, his dad hinted it might be safer for him to marry his younger non blood related sister.....
If thats the case, then did Erika see ghosts?