Complex sentence: Simoun are piloted by priestesses known as (plural; singular "Sibylla"), each a girl who has not yet chosen a permanent sex; doing so renders one physically incapable of piloting a Simoun.
Split into simpler sentences: Simoun are piloted by priestesses known as (plural; singular "Sibylla"). Each "Sibylla" is a girl who has not chosen a permanent sex yet; doing so permanently renders one of being a "Sibylla" and piloting a Simoun.