Is there a sin in shunning a paternal cousin who harbors ill will, instigates problems, and did not attend the condolence gathering for the questioner's father?

There is no harm in shunning a relative who causes harm. However, the mere fact that a relative did not offer you condolences does not permit shunning them. If you endure the harm caused by a relative and maintain ties with them, you will be rewarded and recompensed, in accordance with the Almighty's saying: "Repel [evil] with that which is better, and thereupon the one whom between you and him is enmity will become as though he was a devoted friend."