Jocasta is the queen of Thebes, because she is married to Oedipus, the king of Thebes: "The heralds no sooner reported Laius dead than [Oedipus] appeared and they hailed [Oedipus] king of Thebes" (Line 812-813).
Jacosta's tragic flaw is selfishness: " Listen to me and learn some peace of mind: no skill in the world, nothing human can penetrate the future" (Line 780 - 782) Jocasta does not want to uncover the truth because she does not truly care about the health of the people. She is more concerned about preserving her marriage and public image so that she can have an easy, luxurious life.
Jocasta considers herself greater than the gods: "An oracle came to Laius one fine day...and it declared that doom would strike him down at the hands of his son... There, you see? Apollo brought neither thing to pass" (Line 784-794) Jocasta feels that she is more powerful than the Gods and control her own fate because she thinks that the previous prophecy did not come true, since her son was killed at birth.