Is the fiancée wrong for shunning her cousin, who caused her engagement to be broken and tarnished her reputation, knowing that her uncle refuses to believe any accusations leveled against his daughter?

What your cousin did is forbidden from two aspects: establishing an illicit relationship with a non-mahram man, and transgressing by attempting to separate you from your fiancé. This is what the Prophet, peace and blessings be upon him, forbade when he said: "A man should not enter into a transaction over the transaction of his brother, nor propose over the proposal of his brother, unless he gives him permission." This act causes harm and animosity among Muslims. You must be patient with the harm caused by this woman, for in it lies success in this world and triumph in the Hereafter, as Allah Almighty said: "But indeed, if one is patient and forgives, that is [an indication] of great resolve." Do not grieve over what you have missed of worldly matters, for it may be better for you, and "Perhaps you hate a thing and it is good for you." Boycotting this girl is permissible if it is for the sake of Allah and serves a beneficial purpose.