Why do Antigone and Ismene quarrel as the play opens?

In Sophocles' play "Antigone", Antigone and Ismene do not quarrel when the play opens. Their encounter in the beginning revolves around Antigone informing Ismene about their brother Polyneices' burial denial by King Creon, who declared that anyone attempting to bury him would be severely punished with death. Antigone seeks Ismene's support in defying the order and paying proper respects to their deceased brother through a secret burial. Instead of quarreling, Antigone tries to convince Ismene to join her effort, which becomes a crucial point of tension between the sisters as the narrative unfolds.