When Creon condemns Antigone to death for trying to bury her brother Polyneices, Ismene confesses to Creon that she assisted Antigone:
...Say, didst thou too abet
This crime, or dost abjure all privity?
I did the deed, if she will have it so,
And with my sister claim to share the guilt.
However, Antigone refuses to allow Ismene to confess to something she didn't do.
ANTIGONE: Claim not a work in which thou hadst no hand.
Creon believes that Antigone and Ismene had a hand in trying to bury Polyneices, and he condemns them both to death.
Creon's son Haemon tries to intercede on Antigone's behalf, but Creon refuses to change his mind. The Chorus questions Creon's judgment in condemning both sisters, and Creon agrees to release Ismene, almost as if on a whim and with a wave of his hand.
...These sisters twain he shall not save from death.
Surely, thou meanest not to slay them both?
I stand corrected; only her who touched
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.