My father is behaving like this
About 1.5 years ago my parents divorced. I have lived with my mother and younger sister since then. I am 18 and my sister is 13. This summer, my mother brought her parents from China to Canada, to live with us for a while. When my father found out he went ballistic to say the least. He even showed up at our front door at 1 am and rang the doorbell relentlessly for 15 minutes, waking the entire household (we ignored him).
He gets himself worked up incredibly easily over my grandparents because he is convinced that they are intruders to our family, and that they are the cause for my parents' divorce. My grandparents last stayed with us 3 years ago, and there was a lot of conflict while they were here because my dad did not want them here.
Now, when my sister and I visit his home he ALWAYS tells us how evil our grandparents are and how they have no right to come live with us in Canada. Any other time, he is calling the house non-stop. My sister is naive and she wants to live with him. I will be heading to college in a few weeks and I won't be around to keep check on her when she visits him. I'm worried about what sort of twisted thoughts my dad will throw on her.
I do believe my dad has some symptoms of certain psychotic disorders. His family growing up was quite dysfunctional, with his parents divorcing when he was young and his father committing suicide 8 years ago.
I have little to no close relationship with my dad anymore, and I wish it were different but it can be incredibly stressful and exhausting to have a conversation because he always brings up these topics and I want to defend my grandparents. He then accuses me of siding with them and always finishes off with "one day you will understand".