Two sisters who lived in an area where people treated them unfairly. The older sister did her level best to always make sure her sister is smiling until one day, the older sister was killed by the terrible villagers

This could end up in two ways that I can think of;

The little sister could escape and later end up being a psycho aiming to end the village while always having a smile on her face/wearing a smiley mas

She could aim to be like her sister and make younger kids always smile either in her village or outside if she runs away