More specifically, my cousin's husband and his family, neither of whom I knew very well, deeply resented this decision.

resented 是动词 resent 的过去式。resent 来自法语 ressentir，由前缀 re-（加强语气）和动词 sentir（感受、感觉）组成，原意是"强烈地感受到"，引申为"对某事感到不满或怨恨"。resented 在这里表示"对……感到不满或怨恨"。