Is informing my sister about the content of an exam I previously took considered cheating, given that I know she will take it and that her other classmates know nothing about it, and would I be considered disobedient to my parents if they became angry with me for not informing her?

There is no sin upon you for previously informing your sister of the exam content because you were unaware that she would be taking the test. Your action at that time was permissible. However, informing her again is not permissible, as it involves cheating and undermines equality of opportunity. This falls under the prohibited cheating mentioned in the Prophet's (peace be upon him) saying: "Whoever cheats is not one of us." Your not informing her is not an act of disobedience (disrespect), because there is no obedience to a creature in disobeying the Creator. You should, however, soften your tone when speaking with your parents. As for what she remembers from before, there is no sin upon you for that, as it resulted from an action that was permissible at the time.