Is it sinful to accept a married couple, joined by an 'urfi marriage contract, as tenants, and what is the ruling on accepting unmarried foreigners as tenants, knowing that this is the main source of income?

There is no objection to renting residential units to someone who possesses an Urfi marriage certificate, if the prevailing custom among you is that this document is given for a marriage that fulfills the conditions of validity but has not been officially registered. However, if the custom is that this document is given merely for an agreement between a man and a woman to live together as a couple without a valid Islamic contract, then it is not permissible to rent to them, because they are not married (they are strangers to each other), and doing so would be aiding in sin. It is even more impermissible to rent to mere lovers.