In other thread I've proposed this chainn of events:
That was part of challenge to make Mary Tudor wife of Sigismund Augustus. But let's ignore English part and concentrate on the bolded part instead, maybe with little modification-Anna of Bohemia and Hungary dies from some illness before Elizabeth is conceived, in September 1525. Ferdinand in such situation marry her cousin, Hedwig Jagiellon (born 1513) to keep claims to Hungarian and Bohemian thrones. Marriage could happen around 1529 (Hedwig would be 16 yo). What does it mean?
-perhaps Bona's accident during hunting party in 1527, that resulted in death of her child and nearly killed her would be butterflied away? Question of Hedwig's marriage could make Sigismund too busy to play with Lithuanian bear. So we'd have one additional male Jagiellon.
-another wife for Sigismund Augustus is needed (Bona would be happy without Habsburg daughter-in-law), perhaps Ferdinand's niece instead of Ferdinand's daughter. Dorothea or Christina of Denmark? Healthier wife of Siggy means no secret marriage of Sigismund Augustus and Barbara Radziwił, so less troubles for King.
-other daughters of Sigismund I, as sisters of Queen of the Romans, future Empress, would be more desirable among European monarchs, especially HRE monarchs
-no way Sigismund the Old would let any of his daughters to marry Janos Zapolya. It would be really ridiculous-one sister married to King of Hungary while second is married to anti-King? Also fact, that Ferdinand would be married to Zapolya's niece would have interesting consequences. Zapolya's situation without Sigismund's support is overally worse.
Now my propositions of marriages ITTL-Sigismund I would like to marry his second Isabella to Elector of Brandenburg instead of Hedwig, but Bona would fiercely oppose, Isabella was her only daughter about she really cared and she disliked Hohenzollerns. Alos the fact, that her stepdaughter is queen while there is no king available for her own daughters would not make Bona happy. So
-for Sigismund Augustus-Dorothea/Christina of Denmark or Maria of Viseu
-for Isabella-Francis of Lorraine (OTL husband of Christina)
-for Sophia-Albert V, Duke of Bavaria
-for Anna-Charles Victor or Philip of Brunswick Wolfenbuttel (Catholic sons of Henry V of Brunswick, OTL husband of Sophia Jagiellon)
-for Catherine-John George, Elector of Brandenburg? This marriage would need to happen after death of Sigismund I, Siggy the old would not let his daughter to marry open Protestant. Sigismund Augustus would be more open for this idea. Alternatively-John Albert of Mecklenburg (born 1525), OTL considered by Sigismund Augustus for one of his sisters.
-for Sigismind I's additional son born in 1528 (IOTL he was named Olbracht (Albert) but, being prematurely born he was baptized in hurry, so there was no much time to decide about name, so could be named Casimir or Vladislaus instead, but let's call him Olbracht anyway) some Italian princess. He could be made heir of Bona's Italian duchiess of Bari and Rossano and then live in Kingdom of Naples as Italian Duke (Italian Jagiellons, sounds interesting...).
What about this:
-Anna of Bohemia and Hungary dies giving birth to her first child (that child is Elizabeth of Austria, Siggy's OTL first wife, who also dies).
-Ferdinand Habsburg needs new wife with claims to Bohemian and Hungarian thrones, so he marries Anna's cousin, Hedwig of Poland, daughter of Sigismund I.
-So there is already Polish-Austrian match, another one is not only unnecessary-hypothetical daughters of Hedwig and Ferdinand would be Sigismund Augustus' nieces, (and would be too young). Uncle-niece marriages were still uncommon back then, and Jagiellons never married such close relatives.
-Mary Tudor is relative of Habsburgs, could Emperor Charles V suggest her, when Habsburg girls are not available? Only alternatives are Christina and Dorothea of Denmark-perhaps Sigismund doesn't want to complicate relations with Denmark with marriage of his son with daughter of deposed Danish King) and Maria of Viseu.
-For Jane or whoever is beloved wife of H8 and mum of his son(s) marriage of Mary Tudor in some 'far, far away' country means, that her future offspring would have little chance to ever claim the throne.
That was part of challenge to make Mary Tudor wife of Sigismund Augustus. But let's ignore English part and concentrate on the bolded part instead, maybe with little modification-Anna of Bohemia and Hungary dies from some illness before Elizabeth is conceived, in September 1525. Ferdinand in such situation marry her cousin, Hedwig Jagiellon (born 1513) to keep claims to Hungarian and Bohemian thrones. Marriage could happen around 1529 (Hedwig would be 16 yo). What does it mean?
-perhaps Bona's accident during hunting party in 1527, that resulted in death of her child and nearly killed her would be butterflied away? Question of Hedwig's marriage could make Sigismund too busy to play with Lithuanian bear. So we'd have one additional male Jagiellon.
-another wife for Sigismund Augustus is needed (Bona would be happy without Habsburg daughter-in-law), perhaps Ferdinand's niece instead of Ferdinand's daughter. Dorothea or Christina of Denmark? Healthier wife of Siggy means no secret marriage of Sigismund Augustus and Barbara Radziwił, so less troubles for King.
-other daughters of Sigismund I, as sisters of Queen of the Romans, future Empress, would be more desirable among European monarchs, especially HRE monarchs
-no way Sigismund the Old would let any of his daughters to marry Janos Zapolya. It would be really ridiculous-one sister married to King of Hungary while second is married to anti-King? Also fact, that Ferdinand would be married to Zapolya's niece would have interesting consequences. Zapolya's situation without Sigismund's support is overally worse.
Now my propositions of marriages ITTL-Sigismund I would like to marry his second Isabella to Elector of Brandenburg instead of Hedwig, but Bona would fiercely oppose, Isabella was her only daughter about she really cared and she disliked Hohenzollerns. Alos the fact, that her stepdaughter is queen while there is no king available for her own daughters would not make Bona happy. So
-for Sigismund Augustus-Dorothea/Christina of Denmark or Maria of Viseu
-for Isabella-Francis of Lorraine (OTL husband of Christina)
-for Sophia-Albert V, Duke of Bavaria
-for Anna-Charles Victor or Philip of Brunswick Wolfenbuttel (Catholic sons of Henry V of Brunswick, OTL husband of Sophia Jagiellon)
-for Catherine-John George, Elector of Brandenburg? This marriage would need to happen after death of Sigismund I, Siggy the old would not let his daughter to marry open Protestant. Sigismund Augustus would be more open for this idea. Alternatively-John Albert of Mecklenburg (born 1525), OTL considered by Sigismund Augustus for one of his sisters.
-for Sigismind I's additional son born in 1528 (IOTL he was named Olbracht (Albert) but, being prematurely born he was baptized in hurry, so there was no much time to decide about name, so could be named Casimir or Vladislaus instead, but let's call him Olbracht anyway) some Italian princess. He could be made heir of Bona's Italian duchiess of Bari and Rossano and then live in Kingdom of Naples as Italian Duke (Italian Jagiellons, sounds interesting...).