Is committing a sin a consequence of shunning a brother and his wife who harbor ill will and seek to harm the family and destroy the brother's life and reputation, despite their repeated attempts to cause harm?

It is permissible to avoid one's brother and his wife without a complete severance of ties. Rather, the least amount of connection, such as exchanging greetings and speaking on the phone, is sufficient because he has a right to kinship. It is better to maintain ties with him and treat him well, even if he severs ties with you and treats you poorly. However, if there is a benefit in boycotting him, such as deterring him from his vile actions, then there is no harm in boycotting him. All of this is assuming your brother has not repented from the aforementioned acts of theft and disobedience. If he repents, then it is not permissible to boycott him or sever kinship ties with him.