Who is Aunt Cheryl from If the Witness Lied and what is their importance?

Aunt Cheryl is the step-sister of the Fountain children's mother. She comes to live with the Fountains after Laura Fountain, their mother, dies. Reed Fountain ends up hiring Cheryl and putting her on salary. When she tries to get Reed to marry her, however, Reed starts to look for a housekeeper or nanny so he can ask Cheryl to leave.