How to handle paranoid 81 year old father with Alzheimer's, accusing mother of having an intimate relationship with brother?

I understand that asking for advice can be difficult, but more than anything, I just need to vent and possibly gain a different perspective on the situation we are facing at home. I appreciate any feedback or shared experiences.

My father (81, on a wheelchair) has been dealing with dementia-related issues for about 5-7 years. His decline was gradual, initially manifesting as short-term memory loss, and then progressing to forgetting certain past events. Over the last two years, his state worsened considerably. It started with him becoming very sexually aggressive towards my mother (58), demanding sex, wanting to see her naked, making inappropriate sexual comments to my mother, and stating how beautiful she looks, urging her to show him her genitals, etc... This began to take a major toll on my mother, who is his primary caregiver and takes care of him 24/7. This continued and last year he became completely incontinent, so my mother has to change his diapers. Whenever this happens, he begs for sex, not realizing the absurdity of the situation.

This year, my brother, who lives nearby, began working from their home several times a week to support my mother. My father, even during his healthy years, started to get extremely jealous of my brother spending time with my mother and going out for walks. The situation escalated so much that my father is currently absolutely obsessed with my brother, constantly complaining that he stole his wife, reminiscing about their happier times together, etc... Even when my brother is not in the house, he keeps talking about the situation, being paranoid, and accusing my mother of romantically loving my brother. He has days where he doesn't even sleep and aggressively yells at my mother during the night, stating that she left him, and that it's against the law to marry your own son, etc...
My brother and I are doing our best to support both of them and visit as much as possible, but my father refuses and I suppose can't even understand the irrationality of the situation. My mother (a nurse) refuses to place my dad in a pensioner's facility and I just don't know what to do anymore. I live far away and can only visit them once a month for a few days. My brother is seen as the enemy, which doesn’t help the situation. We have scheduled a visit to a psychiatrist next week, although I suspect the primary solution offered will be medication.

I would appreciate any suggestions or your opinion on the matter. Thank you.