Well, a simple DNA test now could determine if they were related or not. A missing brother/sister might be easier to track down than a couple.
Nowadays I’m sure they could do a check on the materials used for the fillings and maybe match it to a supplier, which could then be narrowed to the distribution area. I would guess that back then the European fillings would have had different standards (mercury levels, etc) than North American fillings.
Ooooh, I bet Pee Wee Gaskins did it. That was during his era and right in the area he used to kill people.
Gaskins killed over 100 people and was one of America’s most prolific serial killers. I’d link you to something, but googling “Pee Wee Gaskins” brought up some seriously not nice stuff.