Is it permissible to congratulate relatives on buying a new house, knowing that they bought it through a usurious bank?

There is no objection to congratulating relatives on their new home, visiting them, and supplicating for blessings upon it, even if the price of the home came from an interest-based loan. This is because the unlawfulness of the loan pertains to the liability of the borrower, not to the specific item in which it was spent.