Final answer:
Roderick Usher feared Lady Madeline's death as it signified the end of his lineage, reflected his own decline, and brought premonitions of dread and doom due to their profound twin connection.
Step-by-step explanation:
Roderick Usher feared the death of his sister Lady Madeline due to a few profound reasons. Firstly, her decease would mark the end of the ancient Usher race, as he was fragile and deemed himself hopeless without his last remaining relative. Secondly, as twins bear a striking resemblance both physically and mentally, her decline and demise symbolized his own deteriorating condition. Lastly, her unusual illness and the potential of prematurely burying her alive, due to symptoms resembling death, haunted Roderick with premonitions of dread and doom.
His fears materialize when the twin's connection leads to a climactic moment where, after being buried alive, Madeline escapes her premature entombment. This event signifies the shared fate the two siblings seem to embody. Her emaciated, struggling form finally leads to Roderick's own demise, thus fulfilling his grim anticipation of his sister's death and significantly impacting his own life. The narrative in Poe's work is replete with themes of fatality, the supernatural, and the profound psychological bond shared between twins.