How can one deal with a father who mistreats his children and threatens to sell their deceased mother's property, wishing to marry and squander the deceased mother's money, while the children suffer from psychological and financial pressures and fear of being undutiful to their parent?

The rights of parents are great, second only to the rights of Allah Almighty. Allah has coupled His rights with theirs in many verses. Even if the parents are non-Muslims and invite to polytheism, one must still associate with them in kindness. The child must comfort his father with his money and spend on him when needed. The father may take from his son's money what he needs, without extravagance or harm. Likewise, children must arrange their father's marriage if he desires it and they are affluent. However, if the father will spend the money extravagantly or cause harm to the child, then he should not be enabled to access his money. The father's contentment in what is right is from Allah's contentment, and his displeasure is from Allah's displeasure.