This shows that the character is very fast and that she likes challenges.
The sentence shows how the narrator likes to challenge herself. That's because she decided to challenge her cousins to a race, testing both her cousins and herself.
In this case, we can say that this sentence is a form of indirect characterization, since it shows a characteristic of the character, by presenting her way of thinking and acting.
More information about indirect characterization at the link:
https://brainly.com/question/11803357