Clarissa is the daughter of Ebony, who is one of Nancy's sisters. This makes Clarissa Nancy's niece. In relation to Billy, since Ebony is his aunt (sister of his mother Nancy), Clarissa is Billy's first cousin.

Emily is Nancy's sister. Since Nancy is Billy's mother, her sisters (including Emily) are his aunts. So Emily is Billy's aunt, and Billy, her nephew.

Elaine is Emily's daughter, and not Nancy's daughter. As far as we know, Nancy does not have a daughter, and only a son named Billy. Since Emily is Nancy's sister, that makes Elaine Nancy's niece, and not her daughter.
Nancy is Billy's mother, and his father left them early in Billy's childhood. Nancy has two sisters, Ebony and Emily, and they each have a daughter, named Clarissa and Elaine respectively. Write about how Clarissa and Emily might be related to Nancy and Billy, respectively. Also, isn't Elaine Nancy's daughter? Provide a very short response.