create 2 characters that are opposites. then make them siblings, make sure to change their personalities just enough to meld them into the same family and their new circumstances. be sure to add some kind of conflict between the two.
examples:
- sister that has to raise younger brother.
- older brother that imposes himself on to younger bother to keep him safe.
- older sister that runs away to join the circus and leaves younger sister to take care of the family.
- a heroic strong older brother, sinister weak younger brother.