Ismene is presented as the ideal of how a woman in Ancient Greek society was expected to be: passive, demure, and obedient. She understands why her sister Antigone behaves the way she does but there's no way in a million years she'd ever consider defying Creon's express orders:
I scorn them not, but to defy the state
Or break her ordinance I have no skill.
Creon is the king of Thebes and as a ruler and as a man he expects his word, which is law, to be followed to the absolute letter. Ismene accepts this without demur. Her own personal feelings simply don't enter into it; as a woman and as a citizen she feels compelled to obey Creon.
Though often presented in the critical literature on the play as being some kind of coward, it should be pointed out that Iseme does actually volunteer to share the same fate as Antigone. Moreover, we might like to consider whether Ismene was the wiser of the two sisters, more level-headed, in her acknowledgement that her family had suffered enough without Antigone's making things worse by her willful act of disobedience.