They tested their dna close to the beginning of the case I believe. They were able to figure out they weren't related but didn't have the technology to do dna tests like today. I could be wrong on the times. Their original dna extraction is probably too deteriorated for them to be able to run through a dna database which is why they haven't submitted it yet.
From the link below:
In 2007, the bodies were exhumed by a team of our forensic investigators, then Coroner Verna Moore, agents from the South Carolina Law Enforcement Division (SLED) and forensic pathologists from Newberry Pathology to get samples to be submitted into the state, national and international DNA databases.
Sumter County Sheriff's Office - South Carolina - Cold Case - John & Jane Doe (1976)