Determining relationships of your DNA matches can be problematic if you don’t have all the relatives of your great, great-great, and great-great-great-grandparents traced down. One place to get some help with that are intestate probate records for relatives who died without children of their own. Siblings, nieces, nephews, cousins, etc. may be mentioned–usually by married name. If recent enough, addresses may also be included.
This case from 1972 documented all the living descendants of a couple who were married in 1881 and died in 1913 and 1932 respectively. A great help tracking down long lost relatives.