In the book by Gillian Flynn, Amma’s friends are Kelsey, Jodes (Kelsey) and Kylie. It looks as if Kylie will not be part of the HBO TV show.
Kelsey will be played by Violet Brinson and Jodes will be played by April Brinson.
Kelsey or Violet Brinson is the younger sister who is 15 years old while Jodes or April Brinson is the older sister and is around 17 or 18 years old. For those wondering, Amma is played by Eliza Scanlen who is 19 years old in real life. A far cry from the 13 year old Amma in the book.