Willem Swerus P., born around 1828 in South Holland, married Suzanna P. (different P.) in 1849. Tragically, Suzanna died around the time she gave birth to their son Pieter in 1852. Willem Swerus remarried, to Gertrude, in 1854. In 1855, Gertrude, Willem and, hopefully, Pieter the First welcomed Pieter the Second into their little family. Alas, the joy of two Pieters running around the household in Lekkerkerk was not meant to last, for Pieter the Second died in the fall of 1856. Two months later, the reduced family circle welcomed Pieter the Third later in 1856.
Their family circle would continue to grow (and, tragically, shrink) over the years, until they emigrated to the United States in 1873. And there, on the passenger list, we find two Pieters (21 and 16 years of age, half-brothers), Huibertje, Lena, ... , and Jacob, the youngest, at age 2. I'm not too sure about the spelling of ... in Dutch on the passenger list. I know in the US census, she would go by "Sadie".
The two Pieters would live for many decades not too far from each other in Passaic, NJ, all appearing in a clump in the City Directory with their numerous offspring. The older Pieter (Suzanna's son) naturalized, got a passport, and traveled many times back to the Netherlands; there is no obvious indication that the younger Pieter did the same.
So let this be a lesson to me. First, I thought there was the one Pieter, with confused birth dates. Then I realized there were two Pieters, and erroneously assumed they were cousins. But when I slowed down and paid close attention to who actually got on the boat and traveled over together in 1873, all became clear. Also, WieWasWie helped out. A lot.