zaidan is your stepbrother, at that time your father was having an affair and your mother decided to divorce your father, long story short your mother married a well-established man and had a son

when your stepfather and your mother were shopping at the supermarket your stepfather said "son, take care of rein mikha, because from now on she is your sister" said your stepfather to zaidan, your stepbrother

at night your older brother entered your room and said "rein I have loved you since junior high school"