Concerned with the potential negative result of telling the king of their true relationship, the commentators wonder how much Esther could have told Achashverosh about Mordechai. According to Rashi, Esther told the king how dear or close Mordechai was to her.
More specifically, the Ibn Ezra writes that Esther was saying that Mordechai was her uncle.
However, the M’nos HaLevi writes that Esther explicitly said Mordechai was a brother to her father, and a descendant of royalty. This last may have encouraged Achashverosh to call for Mordechai.