Just read up on High Conflict Personality - I think this perfectly describes PH.
“A high-conflict personality (HCP) refers to an individual who consistently engages in blame, struggles to manage emotions, and habitually intensifies or prolongs conflicts. Though not a formal clinical diagnosis, the concept is widely used in legal and mental health fields.
Characteristics:
Preoccupation with Blaming: They rarely take responsibility, deflecting fault onto a specific "Target of Blame".
All-or-Nothing Thinking: Situations and people are viewed in extremes (perfect or terrible), leaving little room for compromise.
Unmanaged Emotions: They often react with intense, disproportionate anger or distress.
Extreme Behaviors: They may resort to actions that the vast majority of people would avoid, such as stalking, aggressive harassment, or extreme manipulation
These traits are often linked to or overlap with Cluster B personality disorders (such as narcissistic, borderline, and antisocial personality disorders)
How to Manage Interactions:
Dealing with someone who has an HCP requires a completely different approach from standard conflict resolution:
Use the BIFF Method: When responding to their messages, ensure your communication is Brief, Informative, Friendly, and Firm.
Set Rigid Boundaries: Define exactly what behavior you will and will not accept, and stick to the consequences when those boundaries are crossed.
Do Not Defend or Justify: Arguing or attempting to prove them wrong will usually only escalate the situation.”
I think the RF have the measure of him and although it’s a hideous emotional toll to deal with such a family member if you have a consistent strategy you can bump along from one issue to the next without inadvertently escalating it.