Divergence from OTL starts with Magnus, Duke of Östergötland (born 1542), third surviving son of Gustav Vasa having different personality. ITTL he doesn't suffer from menthal illness, is more cunning, ambitious and religiously indifferent and keeps side of his brother John, Duke of Finland, instead of being loyal to Eric. So in 1562, when John tries to convince him to marry Anna Jagiellon (IOTL John Vasa wanted to marry Anna's younger sister Catherine, but their brother Sigismund Augustus wanted older Anna to marry first, so John convinced Magnus to join his trip to Lithuania and marry Anna, marrying older sister Magnus would 'unblock' younger Catherine, enabling John to marry her. IOTL Magnus eventually resigned from this marriage plans under Eric's pressure) Magnus sees it as big chance to elevate his status. Anna is almost 20 years older than him, but as third son born in Swedish noble family, elevated to royal status just in previous generation, this is still attractive marriage option, that gives him some hopes to get Polish and Lithuanian thrones in the future, after death of childless Sigismund Augustus.
So in late June 1562, John and Magnus meets in Gdansk and then brothers go east, to Lithuania, where they meet Sigismund. Marriage negotiations are successful. John promises his would-be brother-in-law money and support against Moscow, in October 1562 in double wedding ceremony two Vasa brothers marry two Jagiellon sisters in Vilnius. John achieved his goal, but spend all his money and seriously enraged not only his brother, king Eric XIV, but also another monarch-Ivan IV of Moscow also wanted Catherine's hand. John and Magnus could expect serious troubles back home, if not some lucky (for them) accident. King Eric was enraged by his brother's action, which, in his eyes, was treason. Eric's European marriage plans all failed, when his two younger brothers would marry princesses. Few days after double Vasa-Jagiellon marriage, Eric was imagining his brothers being tortured, it was his last tought, at the same moment his horse stepped on the hole, Eric fell from horseback and broken his neck. John and Magnus, informed about Eric's death, came to Sweden as fast as possible during storm season. John takes throne after deceased older brother, while Magnus returns to his duchy in Östergötland. With earlier start of John's reign, Sture family would avoid murders and First Great Northern War, known from OTL, would not happen in the same configuration as IOTL-John would ally with Poland and Lithuania against Moscow, Ivan IV would need to seek for alliance with Denmark (so there would be reverse of OTL alliances). Meanwhile in 1563 only child of Magnus and Anna is born-daughter named Isabella.