Final answer:
Elle was a sister to the deceased, as indicated by the intimate and familial bond described in the passages provided.
Step-by-step explanation:
The passages provided suggest that Elle's relationship to the deceased is that of a sibling. Specifically, the text references the deep connection and shared kinship between brother and sister, even mentioning them as twins. From the descriptions given, such as the closeness of their bond and the impact of her illness and death on the brother, it's evident that Elle's connection to the deceased was deeply personal and familial.