Rainbow Pandemonium

I have a question. Is it normal to get kidnapped and have your sister become a sacrifice? No? Thought so, but sadly that's what happens to me and my younger sister! After being summoned to another world with my younger sister, they declare her the saintess and take her away to be sacrificed in four years' time to save their world from a horde of demons. Now I'm on a quest to solve the mystery of the cursed rainbow to gain the power to save her! Wish me luck cus I'm gonna need it!