Is there a sin upon me if I cut ties with my brother after all his threats and aggressions, and will that affect my reward in the month of Ramadan?

It is not permissible for a Muslim to abandon his brother for more than three days. Whoever abandons his brother for more than three days and then dies, will enter the Hellfire. Allah does not forgive those between whom and their brother there is animosity until they reconcile. One must be patient with his brother and reward him with kindness, for Allah Almighty says: "So whoever forgives and makes reconciliation, his reward is due from Allah." If the brother is an oppressor, there is no harm in avoiding him and maintaining the bare minimum of connection, or abandoning him if there is a شرعية (Shar'i) benefit in doing so, so that he may be deterred. It is permissible to seek one's right from those in authority, for Allah Almighty says: "And indeed, whoever avenges himself after being wronged – those have no cause for blame."