Jocasta is implied as having had an impact on the city of Thebes because she committed an act that is said to have poisoned the land. As we learn in the first play of the trilogy, Oedipus Rex, Jocasta and King Laius were given a prophecy that their son would kill Laius and then bed with his mother. Fearing the prophecy, Laius had Oedipus bound by the ankles and ordered for him to be killed. Jocasta gave the boy to a shepherd who was supposed to abandon him on a mountain top but instead gave the boy away to be cared for, which resulted in the fulfillment of the prophecy. One might say that Jocasta is blamable for not having had the boy killed properly. But even if she is not, she married her son and committed incest, which is immoral in the gods' eyes. Any immoral or impure act was considered to be poisonous to not only the people who committed the act but also to the rest of the community thus bringing more and more grief and hardship. Hence, Jocasta is implied as being responsible for the hardships named in Antigone, such as the death of both of Jocasta's sons and her daughter Antigone.
Ismene particularly reflects on all of the tribulations her family has suffered due to her parents' transgression when she reminds Antigone of how many lives in their family have been lost, begging Antigone not jeopardize her own life as well by breaking Creon's decree. We see her reflect on all of the deaths of her family in the lines:
Think, my sister, how our father
died hated and infamous from offenses
...His wife and mother--
both words at once!--took her life with twisted noose;
then, third, our two brothers in just one day
slew each other. (50-56)
The phrase "offenses self-detected" refers to the fact that Oedipus figured out that he had fulfilled his prophecy by killing his own father and marrying his own mother, and "infamous" refers to the fact that he died hated because his deeds polluted the city, showing us how both Oedipus and Jocasta have an implied impact on the characters and events in Antigone. In addition, Ismene's reference to Jocasta being both Oedipus's "wife and mother" also show how Jocasta has an implied impact on the city.