okay thought of a plot, 

Cass and Nate come to Hogwarts in the middle of the year. were Cass is setted into the house of Slytherin and Nate is put into Gryffindor, this is the first time were they have not been together since they ran away from the orphanage for any amount of time, Cass can't sleep and Nate is always worried about his brother when they are not together.