not save from death.
CHORUS:
Surely, thou meanest not to slay them both?
CREON:
I stand corrected; only her who touched
The body.

Ismene doesn't appear again in the play.

One reason that Ismene doesn't appear in the play after her release by Creon is that Ismene plays no part in the events that unfold after her release.

The second reason is that there were only three actors who shared the major roles in Antigone, as well Sophocles's other plays. After Ismene's departure from the play, one of the actors plays Creon. Another actor plays Eurydice and the Second Messenger who reports Eurydice's suicide. The third actor plays the Messenger who reports the deaths of Antigone and Haemon. There weren't enough actors available to play Ismene (or Antigone) in the latter part of the play.

There is a further Greek legend attached to Ismene after the events of Antigone. Ismene was betrothed from childhood to Atys who fought the civil war that led to the deaths of Polyneices and Eteocles and which is the basis of the plot of Antigone.

Atys was killed in the civil war by Tydeus, who later killed Ismene—at the goddess Athena's instigation—while she was making love with Theoclymenus, a fugitive seer-prophet from Argos.

The legend isn't as compelling as the thought of Ismene living out the rest of her days as the last surviving child of Oedipus, carrying with her the burden of the curse on Oedipus's family as a result of his defiance of the will of the gods many years before her birth.

Antigone initially explains to Ismene that Creon has decreed that anyone caught burying Polyneicês shall be stoned to death in the public square. However, Antigone says that she is willing to bury her brother regardless of the king's threats in order to appease the gods and honor Polyneicês. Ismene responds by telling her sister that she is insane for disobeying the king and refuses to help her, which upsets Antigone, who tells her sister that she will hate her soon. Once Creon accuses Antigone and her sister of disobeying his orders, Ismene attempts to earn her sister's respect by confessing that she also planned on helping Antigone bury Polyneicês. Antigone immediately responds by dismissing her sister's attempts to redeem herself and shuns Ismene. At the urging of the Chorus, Creon decides to spare Ismene's life and sentences Antigone to death. Ismene's fate is not directly stated, but one can assume that she continued living after her sister's death as Oedipus's last surviving child.

I often receive this question from my own students upon our reading of Antigone. I think as modern audiences we are accustomed to all characters finding some resolution at the end of a story. But this is a story of Antigone, not Ismene, and Sophocles may have felt it unnecessary to explain Ismene's future. However, if one reads the trilogy together, Ismene's role becomes clearer, and that may prompt some to wonder of her fate.

Some stories exist of a love affair that ends in Ismene's death, but they are not accepted into the canon of this particular family's story line. Instead, most agree that Ismene lived the rest of her life alone, carrying the knowledge of her family's curse. She must also carry the weight of her own indecision, which led to Antigone's death. Thus, while the other members of the family found some solace in their demise, Ismene continues her torture in life.