Is what I am doing by re-establishing contact with my aforementioned brother correct, and is my refusal of my mother’s commands not to contact him considered disobedience to parents? And if I do not maintain ties with him, would that be considered severing ties of kinship?

What you are doing by maintaining ties with your brother is correct and one of the greatest acts of obedience, because it is part of maintaining kinship ties (silat al-rahim), which the texts have commanded and warned against severing. It is not permissible to obey your mother in her request to sever ties with your brother and abandon him, because there is no obedience to a creature in disobedience to the Creator. Disagreeing with her on this matter is not considered disobedience (ʿuquq). If this leads to her displeasure, you can conceal your relationship with your brother. You must also advise your siblings to be dutiful to their brother and maintain ties with him, and not to sever their kinship for worldly reasons.