Is a person sinful for eating food from someone who deals in usury? And is it permissible to prevent one's wife from visiting her family, who rely on usury in their lives and cause discord between spouses?

The preponderant opinion is that it is permissible to eat from the money of a man whose lawful wealth has mixed with unlawful wealth, because the mixing of money or doubt about it does not render it unlawful. The Prophet, peace and blessings be upon him, traded with the Jews and interacted with them, accepted their invitations, and ate from their food, despite knowing of their involvement in usury and their lack of scruples regarding unlawful gain. As for this man's declaring you an infidel, it is not surprising, for they have declared great imams as infidels. You must teach your wife the correct creed to fortify her, and do not prevent her from communicating with her parents, for honoring parents is legislated even if they are disbelievers.