My characters have the ability to traverse the spirit world, or the dreamtime, or the unconscious, or the underworld, use whatever term you prefer. They are shamans, of sorts, and the ability to do this runs through their family. They gain a special level of power by being twins in that family. The family legacy has been a responsibility to maintain a certain level of order in the world (in order to keep humankind from descending into chaos) by keeping order in the collective unconscious. However, there's a problem. A member of their family has a twin level of power despite not being a twin, and no one can determine why.
More to come later.