Is it considered a sin to abandon someone and prevent them from seeing their wife, due to their continuous attempts to sabotage the marriage and threaten their lives, despite repeated attempts to reconcile?

The man mentioned is your wife's maternal uncle, and he is a perpetual mahram to her. He is allowed to see what mahrams are permitted to see. They must maintain ties of kinship, provided there is no fear of fitna (temptation/strife). If fitna is highly probable, he should be treated like a non-mahram, and kinship ties can be maintained by phone or by speaking from behind a veil in the presence of the husband, if fitna is then avoided. The questioner should strive to maintain ties of kinship; if the other party refuses, then there is no blame upon him.