I don’t know if your brother has a personality disorder. I do think he’s a bully. Why, oh why, is he living with his ill mother? I would think the incident with the dog would be the last straw.
Nowhere is it written that parents have to continue to care for a 21 year old who is disrespectful, mean and manipulative. Millions of kids go through their parents divorce. It’s often not easy, it’s true. But most get on with their adult lives. Your brother is playing on your parents’ feelings of guilt and obligation. He is misusing their love.
With all good intentions, your family is enabling a bully. It’s long past time for calling things as they are. If your brother has money for marijuana, training, and a girlfriend, he probably has money to find a room somewhere. If he can’t be grateful for family support and make solid contributions to the household (both financial and doing chores), he shouldn’t be living off his mom. Give him a deadline for shaping up or shipping out.
Do be prepared for an adult version of a tantrum. Bullies don’t give up easily. He will only take the family seriously if you are serious. If he threatens suicide, call the police to get him to the local crisis team for evaluation for possible hospitalization. Ditto if he threatens to harm anyone else.
I wish you well,