I have a toxic brother who has disrespected every member of my family multiple times including spitting on our mother. Am I allowed to cut ties with him?
Not at all. No matter how bad is your brother or relative, you should never cut ties with him but you should avoid endorsing his wrong acts.
You must keep a link with him through any suitable way of communication and keep advising and guiding him as and when you have a chance.
Wassalam.