Promises

Two sister,who love each other but their personality was different. Sunah,who was bright and always cheer-ish was always happy when its morning. Mooni,who was quiet calm and not such bright like her sister,was always smiling when it's night.They call them,Twin sister of galaxy or Sun and Moon empress. Sunah has golden hair that reach until her knees but Mooni has purple-ish blue that reach until her knees too. One day,they found out something is wrong.And until they remember something important and get punished forever.What was the thing that's was so important?