It's difficult to find similarities between Antigone and her sister Ismene. The main similarity is their thinking about the consequences of their acts. Even in this similarity, they're different. For Antigone looks at both the short and the long terms. She considers the insult to her brother Polyneices, her family, and herself over the denial of god-given rights. She also factors in the impact during life and death of knowingly letting her brother be mistreated. And she thinks about the chilling reception that she may receive in the underworld of the dead if she allows a Theban not to receive god-given rights to proper funerary procedures.
In contrast, Ismene situates consequences within the context of the impact on her legal position and social standing. She thinks everything of being in compliance with the ruler and the powers that be, and nothing of being obedient to the higher moral authority of the gods and to the ties of blood, friendship, and love.