An elderly father's unjustified demand for money from his children

Respected Sir, my father-in-law, who is approximately 80 years old, frequently disturbs his sons and daughters-in-law by making false accusations against them. About eight years ago, he even attempted to kill his wife. As a result, the sons separated their mother and moved her to the upper floor of the house. The father-in-law then tried to climb the wooden stairs (common in villages) with an axe in hand, intending harm, but slipped and sustained minor injuries. The villagers intervened and, by falsely assuring him with a promise of a cheque for one million rupees, managed to reconcile the situation. For the past 15 years, he has done no work of any kind, nor has he provided any financial support to his wife during this entire period. He has five sons, two of whom were married at the expense of the eldest brother. One younger son is still unmarried, and the father shows no concern for arranging his marriage either. I personally kept my father-in-law in my home for three years, but now, due to being misled by others and his own mental instability, he has started demanding one million rupees from his sons, despite them being unaware of this demand.
He now threatens to kill everyone, saying things like “I’ll kill you and also kill myself.”
About ten years ago, the eldest son, in collaboration with his brothers, built a shared house. The father-in-law started selling that house for much less than its worth, selling a house valued at five lakh rupees for just one and a half lakh. Even the amount received from that sale was mismanaged or taken by others. Two years later, he demanded that his eldest son buy a house in his (the son's) name, from which he (the father) could earn rent to support himself. However, after the purchase, the father-in-law secretly transferred the lease of that house to his own name, and now, under others’ influence, wants to resell it again for a low price.
We have no objection to him selling the house, but we strongly object to him publicly insulting us and our wives and daughters in the streets. He abuses us in public, saying that once he sells the house, he will have his rural house turned into a murder plot, and demands his sons arrange a new marriage for him. His sons do not object to the marriage, but he further demands they bear all the expenses and give him a monthly allowance of 5,000 rupees. This is a country where people have to work hard for a living and the responsibilities that should have been fulfilled by him were instead carried out by his eldest son. Yet, in 20 years, he has not given a single penny to any of his sons, nor to his wife, nor does he acknowledge her rights or show any appreciation or respect toward her behavior and personality.
Now the question is:
In light of Islamic Shari’ah, are such demands from the father islamically lawful?
Are the expenses of the marriages of sons and daughters the responsibility of the father?
Are such kinds of continuous demands permissible, especially when the sons are practicing Muslims, perform prayers and fasts regularly, and earn a living through hard labor yet the father keeps burdening them with unreasonable demands?
Therefore, we request you to provide guidance according to Shari’ah regarding how we should deal with such behavior from our father.

الجوابُ حامِدا ًو مُصلیِّا ً

If the verbal and written statements of the questioner regarding the mentioned situation are indeed true and based on reality, then the behavior of the questioner’s father-in-law towards his children and daughters-in-law is highly inappropriate and wrong.
It is absolutely impermissible to squander (waste) the sons' property or wealth merely to fulfill personal desires this must be avoided.
Furthermore, it should be made clear that, although the sons’ conduct towards their father as mentioned in the question is out of obedience, respect, and good treatment, if, in order to prevent him from continuing in wrongful and unlawful actions then they limit their support to only providing necessary financial maintenance (nafaqah) and avoid giving him more, such as transferring houses or shops in his name, then they will not be sinful for doing so.
Also, it is obligatory upon the father-in-law to focus on fulfilling the rights of his first wife, instead of thinking about a second marriage. Otherwise, he will not be free from accountability in the Hereafter.

مأخَذُ الفَتوی

قال اللہ تعالیٰ: وتعاونوا علی البر والتقویٰ ولاتعاونوا علیٰ الاثم والعدوان واتقوا اللہ ان اللہ شدید العقاب (المائدۃ:2الآیۃ)۔
وفی الھدایۃ: النفقۃ واجبۃ للزوجۃ علیٰ زوجھا مسلمۃ کانت او کافرۃ (الیٰ قولہ) فعلیہ نفقتھا وکسوتھا وسکناھا اھ(2/437)۔۔