I think you're right. The age difference between the actors who play Sarah and Bethany is the same in real life as their characters'. Mothers and daughters who have very small age gaps can often pass as sisters.
I don't know why they made her up so heavily since they want the actress to look younger than her years. Ironically though, I think she can get away with playing a 14 year girl old because most of those look 18+ when dressed up to the nines.