Final answer:
The narrator noticed a striking resemblance between Roderick and his sister Madeline, highlighting their shared fate. Additionally, despite her death, Madeline had a faint blush and lingering smile, which contributed to the eerie mood of the story.
Step-by-step explanation:
When Roderick's sister was placed in the coffin, the narrator noticed a few distinct things. First, he observed a striking resemblance between Roderick and his sister, Madeline, as they were twins, and this resemblance seemed to symbolize their shared doom.
Furthermore, the narrator noticed that despite Madeline's death from a cataleptic illness, there was still a faint blush on her face and a lingering smile on her lips - a detail that added to the story's eerie atmosphere. This observation plays a crucial role in the narrative, particularly in the development of the story's fatalistic theme and the later revelation that Madeline had been entombed alive.