Is it permissible to visit Muslims who have bought houses with usurious loans, especially if the visit is for the purpose of maintaining kinship ties?

There is no harm in visiting relatives who engage in usury, and they should be advised to repent from it. There is no harm in eating and drinking from their hospitality unless it is known that the money with which they bought the food is specifically unlawful. Shunning people who commit sins is not an end in itself; rather, the goal is to rectify their affairs, and this may be achieved through shunning or through interaction.