Is it a sin not to return a greeting and not to forgive one's paternal uncle's wife, given her repeated harm and deceit?

If your uncle's wife wrongs you, it is permissible to shun her for only three days, and no longer than that, based on the saying of the Prophet, peace and blessings be upon him: "It is not lawful for a Muslim to shun his brother for more than three nights." Returning her greeting is obligatory. If her companionship harms your religion or your worldly affairs, then it is permissible to cease accompanying her and to avoid interacting with her, without causing offense. This is based on the statement of Ibn Abd al-Barr that scholars unanimously agreed on the impermissibility of shunning for more than three days, unless a Muslim fears that interaction would corrupt his religion or his worldly affairs. Furthermore, responding to evil with good fosters affection and repels the evil of Satan, and forgiving those who wrong you increases honor and dignity, and it is a path to attaining God's forgiveness.