Complex sentence: Sigismund maintained close relations with Erik XIV's brother, John, Duke of Finland (later John III) and in October 1562, John married Sigismund's sister, Catherine, thereby preventing her marrying Ivan IV. Oakley (1993).
Split into simpler sentences: Sigismund maintained close relations to Erik XIV's brother, John, Duke of Finland (later John III). In October 1562, John married Sigismund's sister, Catherine, thereby preventing her marrying Ivan IV. Oakley (1993).