Am I sinful if I help my family find a suitable house, with the predominant assumption that my father will pay for it with Riba (interest)?

If you predominantly believe that your father will buy the house with a usurious loan, then do not search for a house for him, because that is considered aiding in usurious borrowing. Refrain and apologize to your parents, explaining the prohibition of usurious borrowing, and be extremely courteous and gentle with them in your advice.