Morgana is Sadie's mother and Evelyn's sister. She and her husband Henry went on an archaeological exploration and never came back. Their unfinished searches and disappearance are driving factors for Sadie wanting to become an archaeologist. The Main Adventure and Dream of Family have Sadie follow in their footsteps (Dream of Family proceeding at a deeper, more detailed level).
In Two Adventures Together, Marcel says that Sadie looks almost exactly like her mother (of course this is evident from the photo we see at the beginning). Marcel also may have held some attraction for Morgana.