Maybe Cassie is an Imaginary friend of brees.....Bree being so desperate for a friend, her mind just created this Cassie girl...thus being real to her..and no one else...thats why daniale had no idea ...
Maybe she was at the doctors to get a pregnancy test...she might of been having "morning sickness" thats why she apperead all pale and sick looking in her last video "how my parents me ...