Martin, what was the family's religious denomination? I would suggest that you contact the church where Stephan and Margaretha were married in 1887 and request a copy of their marriage record. The marriage record would of course include Stephan's exact place of birth.
As you may be aware, following the First World War, Germany had to relinquish most of the Prussian provinces of Posen and West Prussia to the newly re-established independent Poland. That probably explains what happened to the records you mention. Many of the Germans of Posen and West Prussia left at that time, simply because they did not want to live in Poland. If your mother's aunt was from a small village, it is quite possible that the entire village population decided to leave en masse.
I will refer you to three maps:
Map 1 shows Germany as it was from its unification under Bismarck in 1871 until 1918. You will see Germany's states, including Prussia, Germany's largest state by far, and its provinces. You will note the Prussian province of Posen. Rawitsch (if that is where the Piechowski family were from) was located in the southern part of the province, directly below the "s" in the word "Posen", just the Posen side of the border of the Prussian province of Silesia. You will also note how vast Prussia was. It stretched from East Prussia in the northeast all the way to and including the Rhineland in the west. That's a distance of more than 800 miles! Berlin was the capital of Prussia, and from 1871, of Germany as well.
Map 2 shows Germany's territorial losses following the First World War. As mentioned, Germany had to relinquish most of the Prussian provinces of Posen and West Prussia to the newly re-established independent Poland.
Map 3 shows Germany's states today. Following the Second World War, almost all of Germany lying east of the Rivers Oder and Neisse was given to Poland (with the exception of the northern half of the Prussian province of East Prussia, which was taken by the Soviet Union). The 12 million inhabitants of eastern Germany were thereupon expelled from their homes under horrific conditions. They had to leave everything behind. Close to 3 million did not survive the ordeal. It was hell on earth. The Polish authorities then moved Poles in to repopulate those territories. Very tragic. Very sad. (In 1947, two years after the end of the Second World War, the Allies declared the state of Prussia officially abolished.)
Map 1: http://rootsweb.com/~wggerman/map/germanempire.htm
Map 2: http://rootsweb.com/~wggerman/map/germanempire.htm
Map 3: http://rootsweb.com/~wggerman/state.htm
By the way, if you're wondering where my historical knowledge comes from, I'll explain briefly: I spent six years doing post-graduate studies in history at the University of Munich in Germany.