Is Ismene a weakling, a mere fence-sitter, or does she represent a reasonable position?

I think Ismene is reasonable and understands her place, bowing to Creon’s edict and attempting to dissuade Antigone from her act of rebellion. Ismene’s conversion indicates how her resistance is contagious. When Ismene abruptly interrupts Antigone, saying they cannot bury Polynices, as Creon will put them to death. Basically, she opposes burying her brother’s dead body. This means even Ismene shows her characteristic as coquettish and hyper-feminine according to Antigone, she knows how to reject and express her expressions and her opinions to other people. She knows what she does. When Creon and Ismene talk about the marriage, Ismene sympathize with Haemon by saying “Oh, poor, poor Haemon! To have you for a father!”. She plays a role in order to offer the stability of the atmosphere of this play.

She even tells Antigone that she behaves too impulsively. This draws an implication that she shows how all of the characters act. She prefers to stay safe and this can be observed through her attitude judging Antigone’s behaviour. She goes along with what’s easier or what everyone else is doing, rather than standing up for an opposing action or view.

“I was against your choice and made it clear” a response by Ismene to Antigone shows her fixed principle and her own opinion.