Does Antigone treat Ismene fairly?

It all depends on how much you sympathize with Antigone and her defiance of Creon. If we accept that Antigone is doing the right thing, then her admonition of Ismene seems perfectly justified. Ismene is too frightened of Creon and his wrath to see the bigger picture. Antigone defies Creon because he defies the gods. For Antigone, this is everything, and she simply can't understand why Ismene cannot see this.

On the other hand, if...

It all depends on how much you sympathize with Antigone and her defiance of Creon. If we accept that Antigone is doing the right thing, then her admonition of Ismene seems perfectly justified. Ismene is too frightened of Creon and his wrath to see the bigger picture. Antigone defies Creon because he defies the gods. For Antigone, this is everything, and she simply can't understand why Ismene cannot see this.

On the other hand, if we think that Antigone is in the wrong, then her treatment of Ismene would seem unacceptable. We can easily imagine ourselves in Ismene's shoes and realize that we too would probably react the way she does to Antigone's defiance.

But even if we can sympathize with Antigone's plight, we might still think that she is still a little harsh in her criticism of Ismene. Antigone is clearly a woman of immense courage and strength, and those are admirable qualities indeed. However, people like Antigone often have an intolerance of those not quite as strong or as courageous as them. They find it hard to understand that not everyone can do the right thing, often for reasons beyond the individual's control. This is the situation that Ismene finds herself in. In her own way, she too thinks that she is doing the right thing by not assisting Antigone. In fact, Ismene's submission to the will of Creon is typical of just about everyone in Athens—and certainly the city's women, who accept their socially allocated gender roles.

On the whole, Antigone is a sympathetic character, despite certain all-too-human imperfections. But in showing sympathy to Antigone, it is also possible to extend it to Ismene, who is no less human, no less imperfect.