trying to diagnose my father long read

hi i don’t know if im in the right space but i don’t know where else to talk about this. so my father (50) has this problem its been a problem since his 20s even before he married my mom (49) to make it simple he has this idea that my mother cheated on him with multiple men he knows all these men personally and they have advocated that they haven’t done a single thing and my mom on the other hand has ADVOCATED MULTIPLE TIMES WITH BLOOD SWEAT AND TEARS has even taken multiple lie detector tests and passed but my father refuses to believe any of this. he comes up with such story’s that genuinely sounds believable but if you know my mother she’s genuinely the sweetest person you can ever meet all she does is go to church and take care of her grand children and cook. and has many leg and back problems so mobility is hard for her. the story’s that his brain makes are so detailed like he was there and saw it happen it’s scary. all my life i been afraid of my dad i have 4 older siblings and were all collectively traumatized from the things we witnessed our father do to our mother because of these “thoughts” the countless times that me and my sisters had to take our father off our mother as he’s actively beating her and choking her screaming at her asking for a confession should be enough to put this man in jail but unfortunately in my culture we don’t believe in calling the police on family members (my culture is disgusting) i love my mother more then anything and everyone knows she would never do anything like this. the way my father thinks of her is beyond comprehension to anything my mother is capable of. i think my father is schizophrenic or has OCD he has memory’s of things that never happened and has no proof whatsoever at all but somehow believes all of this and claims he talked to a few people about it and they said it’s true THAT NEVER HAPPENED. before my mom married my dad he was married to another woman (my brothers mom) for almost a year and did the exact same thing to her beat her up multiple times about the same reasons with the same people and same story’s. i don’t know why im even writing this i guess im just tired of seeing my mom in this situation and i want to know what’s wrong with my father. what type of illness is this he’s not even the type of man to get help or go to a therapist or something. i want to send him to a psychiatric hospital just so my mother can feel at peace for once in her life. it’s not always like this tho he can go months without an episode and be a somewhat normal person but then it’s just like a light switch he turns into a whole different person. i know this is long and i probably don’t have the right words i just need help diagnosing my father or advice on helping him stop. me and my sisters tried talking to him about it but he’s literally a narcissist and doesn’t even listen when we talk. literally told us that what we’re saying never happened and we shouldn’t get involved in their drama.