I read this book sometime late 1990s/early 2000s. A young girl went to stay with her aunt for the summer. Her aunt lived in a house that had been in the family for awhile. The main character was staying in a room with a balcony? that overlooked the property towards a stream or river. The house had a tower that the girl was not allowed to go into because the stairs were badly damaged. There was an older man and his grandson who lived on the property who the main character would visit. They would pan for small golden nuggets in the stream/river that the main character collected in a vial. The main character sees the ghost of a young woman with a cloak and lantern outside at night and wonders who she is. At some point she ends up going up into the tower and sees a large portrait in the room of the woman she has been seeing. I don't remember how the reader finds out, but there were two sisters that used to live in the house and one of them had a suitor who killed both? of them. The one the main character sees is buried in the cellar. They are able to dig in the cellar and discover her remains.
Eleven-year-old Kathy, spending the summer with her aunt in her old family mansion, is visited by the ghost of a beautiful woman who died in the nineteenth century.