Is a father's guarantee of his usurious loan considered an assistance to sin, and is the children's refusal to go with him considered disobedience, and does the mother's divorce occur if they do not go?

It is not permissible to assist with an usurious loan, and obeying one's father in this wrongdoing is not an act of filial piety, for obedience is only in what is right. There is no obedience to a creature in disobedience to the Creator, as the Messenger of Allah, peace and blessings be upon him, said: «The Messenger of Allah, peace and blessings be upon him, cursed the one who consumes usury, the one who feeds it, its scribe, and its two witnesses,» and he said: «They are all equal.» Disobeying the father in assisting with an usurious loan is not an act of disobedience (to parents); rather, the sin lies in agreeing with him. As for the father swearing an oath of divorce to threaten his children, the prevailing fatwa with us is that if he breaks his oath, his divorce takes effect. However, some scholars hold the view that divorce does not take effect and he is obligated to offer an expiation for an oath.