The REALTOR® Code: Section 11.

A REALTOR® shall not buy or sell, or attempt to buy or sell an interest in property either directly or indirectly for himself or herself, any member of his or her Immediate Family, or any entity in which the REALTOR® has a financial interest, without making the REALTOR®’s position known to the buyer or seller in writing.

In order to have the information disclosed to the public it must be written in the 'Property Overview / Listing Description' section of a listing.