One might describe Ismene as an unheroic character, but a more sympathetic description would be to say that she is a normal person surrounded by maniacs. Antigone's self-sacrifice is noble, but there is something fanatical and inhuman about her preference for the dead over the living. Ismene is a dutiful daughter and sister, but she is prudent and reasonable, qualities that are in short supply in the royal house of Thebes. Her way of living is that of most people, which is why she is a sympathetic character.
One of several paradoxes in Sophocles's Antigone is that Ismene survives but is forgotten by the end of the play. She quietly fades out of the picture. Her sister Antigone, who is dead, lives on in the memory of the audience. Antigone and Ismene's different reactions to the challenges they face reflect the choice that was said to be offered to several Greek heroes, most famously Achilles: a long, contented, normal life, or a short, painful one with the promise of glory.
Antigone chooses to die and be a hero; Ismene chooses to live and be forgotten. Their different life choices tell the audience that it is difficult and dangerous to be a hero, and perhaps also that it is seldom worth the trouble. If all the members of the House of Oedipus had been like Ismene, no one would have written plays about them, precisely because their lives would have been more tranquil and contented.