What Emil organised commercially, Hugo sold. The two brothers worked in a kind of seamless complement that almost no contemporary partnership matched. The Warstein forge converted from shovel-making to drop forging. New customers came: bicycles, then automobiles, then the great railway networks. In 1916 the 85-ton hammer was installed - the heaviest in any production plant in Germany at the time, requiring every bridge between Soest and Warstein to be reinforced just to get the anvil into the works. By 1937 the Siepmann company was the second-largest drop-forge factory in Germany and one of the most modern in Europe, employing between two and three thousand people across its plants. From 1933 to 1938 Hugo served as president of the IHK Arnsberg, the regional industry chamber, and from 1939 onwards as its honorary president - holding that role through the war years and afterwards.

In 1897 Hugo married Louise Emilie Johanna Lämmerhirt, daughter of Alfred Lämmerhirt the engineer who had helped build the Gotthard Tunnel and now directed the Warstein mining association. The marriage knitted together two industrial families in a town small enough that everyone knew. Louise had been raised partly in Winterthur, Switzerland, during her father's years at Sulzer Brothers; she brought a cosmopolitan polish to a Westphalian household. The couple had three children: Grete, who married a marble manufacturer from Allagen; Alfred, who studied business at Humboldt University in Berlin and later sat on boards including Dresdner Bank and Gerling insurance; and Walter, the engineer-inventor who would inherit the works.

Hugo's philanthropy started small - 5 Goldmark in 1907 toward an equestrian monument in what was then German South West Africa - and grew with the company. In 1916, in the middle of the First World War, he and Emil together contributed over 45,000 Papiermark to philanthropic causes. In 1921 they gave 250,000 marks - hundreds of thousands of dollars in modern terms - to build the Warburg children's home on the island of Norderney, a North Sea retreat for orphans. He helped found the Kriegsstiftung des Kreises Arnsberg, the war-relief charity that aided families of soldiers from the district. He sat on the board of the Warstein savings bank. From 1935 he was on the supervisory board of the Beck & Henkel agricultural machinery firm in Kassel. The Siepmann brothers, in modern terms, were the dominant economic and civic force in their corner of the Sauerland.

Hugo Siepmann died on 4 October 1950 at the age of eighty-two. His elder brother Emil followed him exactly four weeks later, aged eighty-seven. Warstein staged a ceremonial march for both men, hundreds of mourners filling the streets of the town that the brothers had reshaped over six decades. The Siepmann concern survived them - it still operates today, still in family hands, still making valves, fittings, and forged steel components in the same Sauerland town. Hugo's son Walter took over the technical and inventive side of the company; Walter's children and grandchildren run what remains. Schippen-Hugo's nickname does not appear on the company website. But somewhere in Warstein, there is still a Hugo-Siepmann-Straße, and the shadow of the brothers' partnership still falls across the local industry they founded.