Is the money our grandfather gives us unlawful, and is it permissible for us to take it, knowing that he sold the building we lived in to a usurious bank and bought another building in which he housed us?

If the bank purchased the building to engage in a prohibited activity, and the seller was aware of this, then the seller has committed a sin, based on the statement of Ibn Farhun al-Maliki: "Muslims are prohibited from selling instruments of war, and this includes selling wood to someone who will fashion a cross from it, selling a house to someone who will turn it into a church, and selling grapes to someone who will press them into wine." However, if the bank purchased the building for a permissible purpose, or if the seller was unaware of its intention, then there is no harm in the sale. The sin of a prohibited sale rests with the seller, not with the price, and therefore there is no objection to benefiting from the price.