While the keyboard isn't her main instrument, the 庄司祐子 here should be flute/whistle/recorder player Yuko Shoji, as she lists another Haibane Renmei album in her discography, and fellow players on this album Masumi Sakaue and Takashi Yasui mention her on their sites. I even found a couple of concert reports mentioning her playing keyboard live: [1] [2].