Tracking down twentieth and twenty-first century relatives can be problematic, but it is sometimes necessary in order to determine DNA matches or see if living family members may have family information, ephemera, or pictures.
Sometimes a person can find one relative when others seem to go missing. The difficulties are magnified when the relative is a female who may have had “one more” husband than the researcher was aware of. While FindAGrave has its issues (not all cemeteries entries are complete), consider searching for those missing relatives in the cemetery where you’ve found one of them.
On a side note, I’ve sometimes spent just a little time trying to figure out why a relative was buried in an unexpected cemetery. Sometimes that can be a clue that the relative’s spouse had a connection to others buried in the same location. Sometimes.