Complex sentence: Luna, who guides and advises the Sailor Senshi, does not know everything about their histories either, and the Senshi eventually learn that Usagi is the real Moon Princess; the Moon Princess' mother had her reborn as a Sailor Senshi to protect her.
Split into simpler sentences: Luna, who guides the Sailor Senshi, does not know everything about their histories either, and the Senshi eventually learn that Usagi is the real Moon Princess. The Moon Princess' mother had her reborn as a Sailor Senshi to protect her.