Is it reasonable to sever ties with my brother?

I think so. You have many great arguments that would lead to a yes, but for me the one involving your son would be enough. He is cruel to everyone including his own daughter. We have a responsibility to our children to keep them safe emotionally, as well as physically. Your brother sounds toxic to relationships, unfortunately. Be strong. Your mom means well, but has not seen the real picture, yet. It's very hard for parents to come to grips with this about a son or daughter. They can't ever give up hope, but it takes awhile to realize that is her relationship with her son, not yours or your family.Enjoy the siblings that are not toxic. You can't fix your brother. Maybe someday he will wake up and maybe not. This story is as old as time, honey.