So Elizabeth is Maddie's younger sister back story:

When Eli was 2 a gang broke into their house, Eli's mother told Maddie to take Eli and leave. Maddie obeying took 2 year old Eli and ran escaping the gangs member on the way. Some streets down Maddie hid Eli and ran back to their house. Two houses passed and Eli became scared so she crawled away to find her older sis. she came across a women thinking it was Maddie. The women thinking she was abandoned took her in. Back with Maddie she came back to where she hid Eli to find she was gone....

Eli and Maddie do meet again but Maddie does not recognise her as for she suffers from Prosopagnosia also know as Face Blindness.

This disorder makes it so that the person diagnosed will never remember people by face but by clothe choose and style and their voice.