All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
Borrowing money with Riba (interest and/or usury) is one of the grave major sins that are confirmed to be forbidden by the Book of the Book, the Sunnah of the Prophet and the consensus of the scholars and the Prophet cursed the person who consumes Riba and the one who gives it to others to consume. Therefore, it is not permissible to commit this grave major sin except in case of necessity or religiously dire need. In our view, what you mentioned is not a necessity to borrow money with Riba because you have a dwelling place even though it is small and because the disadvantage that you fear does not exist at the present time.
However, the necessity that permits borrowing money with Riba is the case when a person is in difficulty that he cannot repel except by resorting to it [Riba], like not finding a means to feed himself or that he does not find clothes to cover his body or a house to dwell even by paying rent (which he can afford); for more benefit, please refer to Fataawa 86701 and 82102.
As regards what you mentioned about you and your family fearing to be forced to live with your brother and his family in the same small house, then this is permissible in case of necessity. If this happens –while a woman being careful of showing her adornment in front of non-Mahram men –then in our view it is less grave than borrowing with Riba, though in principle, a Muslim woman living with a non-Mahram man in the situation which you mentioned is not permissible even if she has a Mahram with her. For more benefit, please refer to Fataawa 92070 and 88814.
Therefore, you may sell this house and buy another bigger house which has separate amenities even if it is in a far place, and then try to convince your mother in a soft and gentle manner if she is not pleased about the location of the house. It should be noted that you are not obliged to obey your mother about living in a small house which has shared amenities.
If we presume that Allaah predestines that you are all forced to live in that house, then you should be careful of some prohibitions, like a woman and a non-Mahram man staying in seclusion, mixing or looking at each other. Moreover, the women should wear Hijab in front of non-Mahram men and living in these conditions is permissible for necessity. As regards fearing that some problems would happen, then you may solve this by forgiving and being tolerant.
Therefore, we advise you not to resort to borrowing money with Riba and be patient with your circumstances until Allaah makes a way out for you (from difficulty). Also, you may look for someone to lend you money without Riba and you should avoid borrowing money with Riba even if this leads to some difficulty and hardship.
Finally, you should know that if someone abandons something for the sake of Allaah, Allaah will compensate him with something better. Therefore, we ask Allaah to facilitate your affairs and suffice you with what is lawful as compared to what is forbidden.
Allaah Knows best.