Character info

This is my first story so I am sorry if it is terrible. The Sakamaki hear a knock on the door and outside is the man they called their father and a small boy. Karl tells the brothers that the boy was their little brother and that they have to take care of him. What will happen when he is all grown up and a certain sacrificial bride arrives? What will happen when the mukami brothers discover him? Will they fall in love or just stay as brothers?