My parents are divorced and I am living with my mother in my grandfather's house. Life in my grandfather's house is not comfortable as we are 8 persons, and my mother is depressed and sick of it. She is so nervous that I am sometimes afraid that she looses her mind. After 28 years, my father offered to buy a house for my mother and myself. I choose a house and after procedures have almost been finalized and my mother so happy with it, my father told me he is getting a loan from the bank (although he can afford to pay cash). I was shocked, but did not have the courage to disappoint my mother. When I only hinted we might not have the house, she was going to break down. My father will be paying the loan and the interests but anyway my father does not pray and all his business transactions are based on bank interests. But I do pray and I am so scared of God's punishment because everything is in my name and I know very well the Fatwas on Riba in the Qur'an. Please tell me what to do so that God forgives me, and can you say that my case was a case of necessity. I can't stop thinking about it and really need your help.

Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

No doubt that dealing in Riba is a forbidden matter. It is even declaring a war against Allah and His Messenger (Sallallahu Alaihi wa Sallam). So, anybody who deals in Riba deserves the curse and the expulsion from the Blessing of Allah. It is forbidden for the Muslim to deal in it or to help in it as this is considered helping in sin and transgression.
But since what this sister mentioned has already taken place, then there is no harm for them to continue living in this house. But we advise her of the following two things.
First, make sincere repentance from your acceptance of your father buying a house with Riba because the situation you mentioned does not reach the level of dire necessity which is the only thing that permits dealing in Riba.
Second, advise your father to give up the sins he is performing. Tell him in a good manner and with a good style to perform his prayers and to give up dealing in Riba or indulging in any other forbidden acts. Show him kindness and be soft with him.
We ask Allah to guide you to the right path and make you adhere to the truth.
Allah knows best.