Ismene is a clear foil to her bolder sister Antigone. Antigone lives by her moral convictions and the law of the gods. She is not afraid to stand up to male power to do what is right.
Ismene, at least at first, is more concerned with self-preservation than burying her dead brother. As she outlines to Antigone, their entire family has been through quite a bit of trauma. She admits she is weak and can't stand anymore contention. She is much more apt to define herself by her gender—as a woman—than as a human being. She uses her societal position as a woman to save herself from having to put her life on the line, something Antigone does not do. In the prologue, she says to Antigone:
We are only women, / We cannot fight with men, Antigone! / The law is strong, we must give in to the law / In this thing, and in worse. I beg the / Dead To forgive me, but I am helpless: I must yield / To those in authority.
Ismene is more akin to the average person, unwilling to rock the boat, and in this way she highlights Antigone's moral courage.
It is hard not to credit Ismene with some common sense for not wanting either of them to stick their necks out for a brother who is already dead after there has already been so much tragedy in their family. However, the gods, as the chorus explains, side with Antigone: one must do the right thing, regardless of the cost. As Antigone says:
But I will bury him; and if I must die, / I say that this crime is holy