Is Ismene right for not helping her younger sister Antigone?

You could argue either way on this question. Legally, Ismene is right because she is following the king's law. Ethically, though, you could argue that she is not right and that Antigone is right to want to give her brother a proper burial.
Since Oedipus was banished in the previous play—the first of the trilogy Oedipus the King—Creon, his brother-in-law and uncle, serves as king. Oedipus's daughters, Antigone and Ismene, are living in Thebes, and Creon has vowed to watch out for them as they grow up, as Oedipus knew how difficult their lives would be and he even believed the family was cursed. In Antigone, Oedipus's two sons have died in battle. Creon has allowed one of them to be buried properly, but the other son, Polynices, is believed to have fought for the "wrong" side and therefore to be a traitor to Creon and Thebes. Creon decrees that Polynices will be punished by not receiving a proper burial, and Creon forbids any to bury him.
Antigone, though, feels her loyalty to her brother is stronger than her loyalty to her king. She believes it is ethically wrong to deny someone a burial. Therefore, Antigone takes it upon herself to bury her brother. Her sister Ismene does not help her. Antigone ends up imprisoned and eventually dead.
While Ismene follows Creon's decree and does not break the law, the audience might see her as weak and ethically wrong. Antigone is the heroine of the play, and even though she acts illegally, the play suggests that the decree is unjust and that Antigone is correct to bury her brother.