Well, there are also stories that say that ghosts are bound to people, rather than places. If the haunted person moves to a new location, the ghost is moving along, to continue its haunt.
Of course, even this theory has its problems, considering Jeremy is dead. So the question would be, is Emily haunted by one of her ancestors and the powers of Derceto has corrupted the ghost?
If so, why is the ghost still there, when Edward is the player?
Does that mean, Emily was haunted as a child? The picture and her introduction do state, that emily must have liked her uncle. There's a picture of him and her, as you said, and the death of Jeremy was a shock for her. And since she doesn't believe of him having comitted suicide, but was killed, speaks for itself.
In any case, if Emily was haunted and liked her uncle, she must have visited quite a lot, which might have ended up trapping the ghost that followed Emily.
Or, as you said, Jeremy is a widower and the ghost is his wife.
Also, there's no statement, that Jeremy and Emily are blood-relatives. Yes, they are both called Hartwood, assuming Emily is still single. But for all we know, Jeremy's wife could have had a sister, who married Jeremy's brother. At least I can't remember Jeremy's sibling ever being mentioned. He is Emily's uncle, so he must have had a brother or a sister.
Most likley a brother, considering the wifes took their husband's family name back in the day.
Or Emily is married and her husband is called Hartwood, making it possible, that she's a blood relative of Jeremy', but his late wife and Jeremy used her as a substitute of not having children of his own.
Of course, this is just mere speculation, but where's the fun if we don't make up theories about who this ghost is?