Writer's block buster

Idea #1

Brother and Sister (names tbd) are living happily until one day Sister gets abducted to the Underworld, Brother, fearing for Sister's life, goes to see (some priest or something?) s/he informs Brother that in order to get into the Underworld you must be: dead, a demon, taken there by a being of a higher power, a holy figure, or have been there before. Brother thinks for a bit before telling them to turn him into a demon, they reluctantly comply. After being turned into a demon, Brother descends into the Underworld and must battle other demons hindering his path to Sister, much later Brother finds Sister and kills the demons holding her hostage, however as he is currently a demon she doesn't recognize him as Brother and goes back to the Surface. Upon returning Sister hears from (priest person ) that Brother had gone into the Underworld looking for her, after this, Sister returns to the Underworld and finds Brother, having kill all the most powerful demons, is now hailed as the demon king (no idea after that though)

So any thoughts?