Ismene fears for her sister's life because Antigone expresses a fervent desire to break a new decree of Creon's. A civil war has just been fought between the two girls' brothers, Eteocles and Polynices. Since Eteocles held the throne at the time, Creon views Polynices as the traitor to the state and Eteocles as the hero. Therefore, Creon decrees that only Eteocles will be given an honorable burial, and Polynices will be left unburied and unhonored. Creon is so earnest about preventing Polynices' burial that he has announced that anyone who tries to bury Polynices will be stoned to death. Hence, Ismene fears for Antigone's life because Antigone tells her that she intends to bury their brother.
Antigone desires to bury her brother because she feels that Creon is unjustified in forbidding even Polynices' own closest family members to give him a proper burial. We see Antigone express this sentiment to Ismene in her lines, "Such things they say our good Creon decreed for you and me--for me, I say!" and even further still in her line, "He has no right to keep me from my own" (31-32, 49). In addition, Antigone also desires to bury he brother because she sees it as the unwritten command of the gods to honor the dead. She sees it as more important to revere the gods than to revere Creon and that the gods' laws are far higher than Creon's, as we see in her lines, "...since I must please those below a longer time than people here, since I shall lie there forever" (74-76).
Hence, we see that because Creon has promised death to anyone who buries Polynices, Ismene fears for Antigone's life because Ismene knows of Antigone's desires to go against the command of the state by burying their brother.