Is it permissible to accept an invitation from a relative who works in a riba (usury or interest) bank?

Question:
May Allah reward you, Honorable Shaykh. This questioner says: If my uncle works in a riba bank, and my aunt works in a permissible job, and one day she invited me to dinner at their home, should I go to their home to accept the invitation?

Answer:
Yes, if you do not know that this money is from what is forbidden, then you eat from the food, because the default is that it is lawful except for what you know to be from what is forbidden, in which case you do not eat.