Prequel: Mitasareru Kuchibiru.
POV switches between uke and seme.
The back story of the oni-san, is in the prequel. The prequel is manly about the little brother. Not entirely necessary to read that, to understand the this. I like this story betteral.
Although it is a good ending, wish it was more love ly dovey and the seme actually saying his love back to the uke.
Sasayaku no wa Sono Yubi