The detail in the passage that provides a clue to the meaning of the French word is:
<em>"I just got invited to three different birthday parties!"</em>
This shows us what the reason is for Ava's sister to call her <em>populaire</em>. She got invited to three different birthday parties, which means that there is a lot of people who like to spend time with Ava. In other words, she is very popular.
The six letters that appear without interruption in both <em>populaire</em> and the English cognate for the word (<em>popular</em>) are: p-o-p-u-l-a