So I'll avoid using the words "common" and "uncommon" because I don't have any statistics to show exactly how common it was for brothers to serve in the same unit. That being said, it was definitely a possibility mainly due to a German recruitment method known as "primary grouping". Primary grouping was the German attempt to install a sense of cohesion and loyalty in the German army. Under this system German soldiers would be drawn from roughly the same conscription zones (Wehrkriese). This meant that troops were trained and grouped roughly on a territorial basis; also when a man got injured he would most likely be returned to the same unit after he was recovered. The idea was to make soldiers see their unit as a second family and develop a sense of loyalty and camaraderie with their fellow soldiers and officers. This got harder to do at the divisional level and so was more of a platoon/company level occurrence. Now as the war dragged on an manpower got scarce these groups generally got broken up. A lot of historians used to point to this system of grouping as being of the German army's greatest strengths (and its destruction helped signal the end of the German army as a feared fighting force); but this view isn't all that common anymore.

Anyways, so definitely two brothers fighting in the same unit was certainly possible; again I don't want to use common cause I don't have the statistics on hand (truth be told I'm not sure such statistics would exist in the first place). Sorry if this isn't exactly the answer you were looking for, but hopefully it helps shed some light on the situation.