Is it a sin to help a relative pay off a Visa card (which is suspected of being un-Islamic due to financing and late fees), and does lawful money mix with the installment money when deposited into my account?

There is no objection to using a Visa card if it is free from the conditions of charging interest or a penalty for late payment, or taking excessive issuance fees beyond the actual cost of the uncovered card, or taking an excessive percentage on cash withdrawals from the uncovered card, or purchasing gold, silver, and currencies with an uncovered card. If the bank charges a commission for late payment, then it is an interest-based card, and it is impermissible to use it or assist in its use. You must advise your relative to cancel it and refrain from helping him pay it in the aforementioned manner. Your money mixing with his money does not cause harm, because the prohibition relates to usurious borrowing or dealing with the interest-based Visa card itself.