In the Greek play "Antigone", why does Ismene plead Antigone not to defy Creon.

Ismene, Antigone's sister reminds her that they are both only women, and therefore, they are powerless in their society.  Although the society was a democracy, it was liminted to men, women had no rights, no say, no authority under the law. 

"Democracy was practiced differently in Ancient Greece than it is in the modern United States. Full citizenship, which included the right to vote, was only given to free men; women and slaves were not considered full citizens and so lacked the same rights as men. They were forced to follow a different code of conduct. Despite such inequities and restrictions."

The King, not only a man, but a powerful authority figure with the ability to put someone to death for defying his orders, is not someone to go against Ismene tells Antigone.  She also loves her brother Polyneices, but believes that obedience to the king is more important that burying her dead brother.

"She reminds her sister that according to their role as women, it is not for them to decide what is right or wrong. When Antigone is caught, Ismene is willing to share the punishment, but Antigone denies her sister's involvement."

Even though she does not agree with Antigone's plan, she believes her sister to be courageous.  Antigone is determined to go ahead with her plan, defy Creon, at the risk of losing her own life.  Antigone is motivated by love for her brother and by her religious beliefs. 

Antigone believes that Creon has put himself above the will of the gods, taken upon himself authority, the decision to leave Polyneices unburied,  that he does not have, he cannot defy the gods and leave Polyneices unburied.  If Polyneices remains unburied the gods will become angry, displeased with Thebes and Polyneices will not begin his journey into the Underworld.  He will not be permitted in the afterlife.  So Antigone believes that the will of the gods is more important that being obedient to the King.  She must help her brother reach the afterlife. 

But Ismene does not agree, she is fearful for her sister's life, you can't blame Ismene.  She is in the midst of grieving the loss of her two brothers, Eteocles and Polyneices who fought in battle and killed each other.  The last thing that she wants to see happen is the death of her sister.  She has no family left, both her parents are dead, now her brothers, and soon her sister.