We never saw Laura onscreen in the 1700s, but we find out in 1897 that she had been Jerimiah's first wife, when Barnabas was a child. It's rewritten plot, because originally with Laura in 1967, her history was that she married into a different family each generation. In the 1897 story, they changed it so that Laura always married into the Collins family, generation after generation. So in order, she married Jerimiah Collins, Edward Collins, and Roger Collins. Except if you're watching Laura in black and white, in which case Roger was the only Collins she snagged. History was changed, without any time travel from Barnabas!