The modern director (of Sophocles' Antigone) has to worry about so many things which are completely unimportant, things which did not concern the orginal actor who was a man in a mask. How old is Ismene? Is she older than Antigone or younger? Or a twin (like Eteocles and Polyneices)? Sophocles does not care about any of this, nor what she would wear, how she should speak etc. But your decision on any of these will affect the way the play comes across. Is she prettier than Antigone, for example? Would you have her wear a dress and long hair while Antigone has a skinhead, DMs and levis?
There is nothing in the play to help you here at all! As Sophocles wrote her character, she is the negative to Antigone's positive (or perhaps better the positive to Antigone's negative). She is the norm - Ismene is what a Greek girl is supposed to be like (and Antigone isn't). She believes men are stronger, that authority should be obeyed, that you can't beat City Hall, and that a girl should keep a low profile (especially if her family is already notorious for its freaks). She would fulfil Pericles' blueprint for the ideal Athenian woman (in Thucydides 2. 46):
"the greatest glory for a woman is not to be discussed by men at all - whether they are criticising her or admiring her."
Ismene does not want to be noticed, or draw any sort of attention to herself. "A modest silence is a woman's crown" as another writer puts it (Euripides' Andromache).
So much for her first scene - Sophocles brings her back later purely to highlight Creon's unreasonable cruelty (Antigone is a terrible role-model for a young girl - but amazingly she happens to be RIGHT) - we know Ismene's innocent, but Creon condemns her anyway - at first. Later he relents, and this is the first sign that he is going to crack up, and try to go back on his wrong decisions. Unusually for Greek tragic characters, Ismene in her second scene now has to play a different role - dramatically her task is to introduce the theme of Antigone's betrothal to Creon's son Haemon. This is not character development - she is still subordinate to the dictates of the plot. If Ismene was not there, it would be hard for Antigone to do it without introducing an irrelevant "lurve" theme. Whether they love each other or not is not important to Sophocles : Haemon is like Ismene, the perfect Greek young man, whose duty first and last is to his father and his family. The heart of the play is that confrontation between father and son - because this was a matter of real importance for Athenians.
So - my advice - make her as normal as you can, to highlight Antigone's extraordinary behaviour. I think it would be a very powerful way to do the play (and original!) if in the first scene you tried to make the audience side with Ismene completely. We should see that Antigone is stupid, illogical, and abnormal - which makes it all the more exciting when it turns out she is right!