Is the brother's wife considered a next of kin, and would one be sinful for ceasing to visit her and avoiding her due to her bad behavior and harm?

The brother's wife is not among the kinship ties (Arham) whose maintenance is obligatory. Kinship ties are the relatives from a person's roots, branches, and collateral relatives who are connected to them by a common womb. However, the brother's wife has rights like all Muslims, and she is closer due to being the brother's wife and the mother of his children, which necessitates maintaining a good relationship with her. Therefore, we advise patience, endurance, initiating greetings, forgiveness, and repelling evil with what is better. If these efforts do not succeed and you fear harm from her, there is no sin in abandoning her gracefully by giving her her due greetings and limited interaction without close mingling, as abandonment is permissible if close mingling leads to a deficiency in religion or harm.