Sophocles' Antigone presents us with a number of interesting pairs: Polyneices versus Eteocles; Creon versus Haemon; and Antigone versus Ismene, just to name the three most obvious in this play.
Sophocles likes to bring pairs on stage together to draw out the contrasting sides of the issues that are central to his plays. In the Antigone, one of the central issues has to do with laws established by humans and laws observed by the gods.
For Antigone, burial is a right that "the gods all hold in honour" (Ian Johnston translation). In contrast, her sister Ismene feels that human beings in positions of power should be obeyed even in situations "which bring us...harsher agonies".
Thus, I would say that the main purpose of Ismene in this play is to provide a contrast, or foil, to the views of her sister Antigone. Compare also Sophocles' Electra, where the playwright has sisters Electra and Chrysothemis debate the merits of avenging their father Agamemnon's murder.