Ismene has several purposes in the play. The first is simply that she is part of the story. Unlike modern plays, Greek plays were not made up ex nihilo but rather based on existing epic poems (most of which are, unfortunately, no longer extant). So playwrights mainly worked with materials they took from other sources, with an already fixed set of characters.
Ismene is a fairly minor character in the story but Sophocles uses her as a foil to Antigone. In many ways, for Sophocles, Ismene is far closer to the ideal of Greek womanhood than Antigone. She is concerned for family, like Antigone, but less aggressive and arrogant in making choices, and sees her herself as acting primarily in a supporting role in the domestic sphere, something that would have been considered the correct behaviour for women of the period.