Is it permissible to buy a house with money sourced from bank interest if it is the only means of housing for a needy family?

Usury (Riba) is forbidden, and those who consume it have been warned of war from Allah and His Messenger. It is one of the gravest sins. Your aunt should be advised to avoid usury and dispose of any money acquired from it. As for the house, it is not permissible to accept it if your aunt will pay for it from forbidden usurious interest, unless there is an urgent necessity for housing that permits what is otherwise prohibited. However, if she will buy it from her mixed wealth, then there is no harm for you in taking it, whether you are poor or not.