As Oedipus' other daughter — the more prominent being Antigone — Ismene represents primarily a complement and contrast to her sister.
In Oedipus at Colonus, Ismene serves her father mostly as an information gatherer, a resident of Thebes who can bring her wandering father and sister news of their home city and the rest of their family. Note that she makes a memorable entrance in the play, riding a colt and wearing a large sun-hat. As Antigone's sister and Oedipus' daughter, Ismene is an especially important character in the drama. Possibly Sophocles chose this unique entrance as a way of marking her as one of the members of the family, rather than just another passerby in Colonus.
Both Ismene and Antigone represent filial duty in Oedipus at Colonus, but Ismene takes the less heroic role. Unlike Antigone, Ismene, it seems, has a nearly normal, stable life. She does not serve her father's needs or share his danger daily, as does Antigone. Although Ismene's devotion obviously exceeds her brothers' — even her father praises her to Polynices — it does not equal the sacrifice of Antigone. Ismene is continually in her sister's shadow.
In Antigone, Ismene's fear of challenging Creon and the laws of the state prevent her from sharing in her sister's bold plan and, ultimately, her fate. In this play, the closest view of the sisters' relationship, Ismene's words and actions make clear that she loves her sister greatly, but differs from her greatly, too. Unlike Antigone, Ismene seems paralyzed by her cultural identity as a woman. Ismene is emotional rather than passionate, more likely to plead for mercy than demand justice. The last survivor of Oedipus' house after the death of Antigone, Ismene nonetheless seems to vanish at the end, her identity lost in the culmination of the tragedy.