Many details above, but, past history with this sibling and I has always been contemptuous and mutual dislike. Sibling has history of documented anger problems, but is a high dollar sales person and will never admit being wrong. Disliked by many people in community, but has no idea. Self righteous and sexist and bigot. Feel like I can’t do anything as parent will always take up for this person.
Is Dad living with you? If so, ban your sibling from YOUR house. Tell him/her when he can respect you, Dad and your home then he/she can come home.
If Dad living in his own home, then he needs to stand up to this bully. So what if he blows up. Have someone there when Dad tells him that he/she is not welcome in his home if he can't respect him. If it gets out of hand, call the police and then file a restraining order. Dad is #1.
As a mental health therapist you must realize your sibling has a mental disorder that needs to be addressed. No one needs to be the brunt of her/his abuse and that is what it is.