Although Jahnke is not an uncommon name from Northern Prussia, I have noted with interest some names from Southern Minnesota that have common first and middle names. Since I wrote this I have obtained proof (via a Birth recorded in a Registry from the late 1870's from Reinwasser) that my grandfather Wilhelm Julius Jahnke was born in a small town called Jacobshausen close to Reinwasser to Johann Jahnke and Frederike Marutz. I have no further evidence or oral tradition on Johann's father or siblings. However, interesting your family has similar first names. But that is a far as we can take it for now. Thanks for your interest and if you ever run across something that shows a relation please contact me again.