Is it considered a sin to cut ties with the paternal uncle, his son, and his grandson who assaulted the family land and insulted the mother, knowing that reconciliation attempts have failed?

Maintaining kinship ties (silat al-rahim) is obligatory for mahram (definition) relatives and recommended for non-mahram relatives. Severing these ties is forbidden. If the uncle is the one who severs the ties, there is no sin upon you. However, you should strive to maintain ties with him and not reciprocate his severance, for the one who maintains ties is not the one who reciprocates, but rather the one who maintains ties with those who cut him off. Whoever maintains ties with his relatives who cut him off, and treats them kindly even if they treat him poorly, will have support and assistance from Allah. "You will continually have a supporter from Allah as long as you are upon that." As for the paternal cousin (ibn al-'amm) and his son, maintaining ties with them is not obligatory, and it is not permissible to abandon them for more than three days except for a valid reason. Merely greeting them is sufficient to end the abandonment. However, abandonment is permissible if its cause is a sin and it is hoped that the abandonment will deter them from that sin.