In Sophocles' Antigone, why does Antigone feel that Creon and Ismene are all planning her fall?
I would actually hesitate to say that Antigone thinks that Ismene and Creon are planning her fall. They are, however, refusing to side with her, which, in the case of Creon, leads to her death.
We see Ismene oppose her sister Antigone when Ismene refuses to help her bury their brother. Ismene refuses to help her sister for several reasons. One reason is that since Ismene has already suffered so much death and destruction, she simply can't bear the thought of any more. As Ismene points out, they have already lost their father, their mother to suicide, and now both of their brothers; she simply can't bear the thought of endangering her own life as well. Another reason she refuses to help is that, as she argues, as women they are far weaker than men and the law; therefore, they should simply act their part and submit. We see the reasons behind Ismene's refusal expressed in her lines:
Rather, consider that we were born women, proving we should not fight with with men, and that we are ruled by more powerful people and must obey them
to do this against the state--I have no strength for it. (61-64, 78-79)
By refusing to help her sister, Ismene is not necessarily planning Antigone's fall, but she is refusing to support her.
Creon also sides against Antigone in refusing to believe that the gods' laws are higher than his own and that she ought to be allowed to bury her kin. Instead, Creon argues that mortals cannot pollute the gods by dishonoring them, which he says to Tiresias in the lines, "I know well that no human is strong enough to pollute the gods" (1049-51). He further argues that Polynices should be judged as a traitor for trying to overthrow his brother and that the "wicked" should not be honored "equally" with the good (532). Hence, since Creon opposes Antigone with his own arguments, he becomes responsible for her downfall by sentencing her to death.