It's unknown why after his death in 1312, his father left Siemowit the most extensive part of his Duchy, Bytom (already since 1311, he was made the formal co-ruler).
Despite Siemowit's role that was not too important, he established contacts with the court of Władysław I the Elbow-high in the Wawel Castle.
Siemowit could maintain his rule over Bytom for only four years, until 1316, when, in unknown circumstances, his older brother Władysław assumed the government of the Duchy.
The next information about Siemowit is found on 19 February 1327, when, together with his brothers Władysław and George, he paid homage to the Bohemian King John of Luxembourg in Opawa.
This event demonstrated that, whatever the reasons why Siemowit lost his sovereignty over Bytom in 1316, at the end of the 1320s there is a full reconciliation between the brothers.