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Allegedly Bona said, that her oldest daughter Isabella should be born as boy instead of Sigismund Augustus. Isabella was her only daughter about she cared-she saw her as continuation of her own person and had ambitious plans about her future, she wanted Isabella to be queen (and she achieved her goal, although not exactly like she planned: instead of French Dauphin Isabella married anti-King of Hungary, over 30 years older than her, who soon died, leaving her widowed with infant son in unstable foreign country).
So let's say that, like Bona wished, her firstborn child is a son. Boy, named Sigismund (but not Sigismund Augustus-unlike Bona's OTL firstborn son he is not born on 1 August) was born 19 January 1519, exactly 9 months after wedding of his parents. So Bona's ambition would not suffer like IOTL. She would think that she proved to be better queen than Barbara Zapolya. Birth of the next son in 1520 (for sake of simplicity let's say, that rest of her children are roughly like IOTL, with one exception-there is no accident with the bear in 1527, although Bona's last child is short living anyway) further improved her self esteem. Things are bit different from similar "Sigismund Augustus has younger brother" scenarios.
Within decade serious changes would occur, not only in Poland and Lithuania.
-ITTL Bona's oldest daughter is born in 1522 (and she is the one named Isabella, followed by Sophia in 1523 and Anna in 1526). That could have impact on remarriage of Francis I of France. IOTL anti-King of Hungary Janos Zapolya (whom France, alongside Ottomans, supported against Habsburgs) tried to arrange marriage of his niece Hedwig (born 1513) daughter of Sigismund the Old and Barbara Zapolya with French King. Francis preffered Hedwig's half-sister Isabella, due to her Sforza blood and rights to Milan and suggested Hedwig's marriage with his ally James V of Scotland, which was not something King of Poland was interested in. But ITTL Isabella is not 6 but 9 years younger than Hedwig. Francis may think that it is not worth to wait for her, better leave her for one of his sons. So perhaps around 1529 Francis would marry Hedwig? Politically that would make sense (creating alliance with Zapolya and Jagiellons against Habsburgs).
Other possible changes: Vivente Rege is unlikely. There are two sons of Sigismund I of similar age, so in theory there is choice between the two. IOTL Sigismund Augustus was only legitimate son of his father, so it was obvious anyway that he would succeede dad, that is why VR was possible at all. Still, older would be made Grand Duke of Lithuania during father's lifetime, while younger would be send to Italy to rule in Bona's duchies of Bari and Rossano.
Daughters of Bona, as sisters of French Queen, should be married earlier and better than IOTL.
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