Complex sentence: Frances dies giving birth in June 1778 to Hareton, which results in Hindley's descent into a life of anguish and inebriety, so Hareton is cared for and nursed by Nelly Dean, the sole narrator of the story.
Split into simpler sentences: Frances dies giving birth in June 1778 to Hareton, which results in Hindley descending into a life of drunkenness and anguish. Therefore, Hareton is cared for and nursed by Nelly Dean, the sole narrator of the story.