Elma not her sister?

When Elma first gets on the bus I'm sure she mentions going to find/visit a sister in the city. Cherie also later mentions she and her baby sister left there home town due to flooding and she grew up to get a job at a cafe or rest stop (not sure exactly what she said now) but I was always under the impression that they were sisters but hadn't seen each other in a long time and Elma was going to find Cherie in the city. Obviously I was wrong as they didn't recognise each other on the bus or even mention anything about being it. But does it not seem slightly odd that this mention of sisters was brought up only to lead to nothing? Did anyone else have the same thought I did?