Complex sentence: In the late 1250s, Jacob Svetoslav was already an influential noble and married a daughter of Theodore II Laskaris from his marriage with Tsar Ivan Asen II's daughter Elena.
Split into simpler sentences: In the late 1250s, Jacob Svetoslav was already an influential noble. He married a daughter of Theodore II Laskaris from his marriage with Tsar Ivan Asen II's daughter Elena.