There are a couple of different contrasts that emerge between Antigone and Ismene in the opening of Sophocles' Antigone.
The conflict between these two sisters, these two daughters of Oedipus arise when their two brothers, Eteocles and Polyneices, are killed in battle. Eteocles was fighting to defend Thebes; Polyneices was fighting to regain the kingship of Thebes. After the two brothers die in battle, Creon, the king of Thebes, decrees that Eteocles will receive a proper burial, while Polyneices must not be buried.
This decree causes the conflict between Antigone and Ismene. Ismene feels that defying Creon's human law will result in significant problems for the two of them and she argues that as women "we shouldn’t fight with men" (Ian Johnston's translation).
Antigone, in contrast, wants to bury Polyneices becuase she feels that proper burial is a right sanctioned by the gods themselves and that no human law can take precedent over divine law. According to Antigone, "for those laws the gods all hold in honour" (Ian Johnston's translation).
Thus, two of the major contrasts that emerge from the opening of Sophocles' Antigone are the contrast between human law and divine law and a contrast regarding the role of women in society, especially with respect to their duty towards the city and their duty toward their loved ones.