That’s a good question, one that Sophocles does not answer for us in Antigone. Ismene’s final appearance in the play happens in the middle as she asks Antigone to let her share her fate, which is death at Creon’s pronouncement. Although Ismene is Antigone’s sister, Antigone is harsh with her, not wanting to allow her to suffer the consequences of a righteous deed (the burying of her brother) which she did not perform.
No, sister, do not dishonor me, but let
me die with you and honor him who died.
You may not die with me, nor call yours that
which you did not touch. My death is enough.
Ismene realizes that without her sister, she has nothing left to live for, so Sophocles simply lets her disappear and moves on to other things. The implication here is that someone who does not risk their own safety to do what is right is not worth being concerned about.