Biological relatives are the logical place to begin your search for family history information. And while biological relatives are the obviously place to search for others who may be willing to take a DNA test, non-biological relatives may be the place to get additional information.
If your ancestor/relative was married several times, the descendants of the final spouse may have family history items from your family in their collection of old materials. Descendants of ancestral associates may have information about your relative. Former neighbors of somewhat close family members may have stories about your family member that your immediate family do not know or are not willing to share.
One way to locate some of these individuals is to reach out to local history groups on Facebook. In other cases, it may be possible to track down descendants of a relative’s “last spouse” to see if they have pictures or other items.