Juana de Castile, only daughter of Enrique IV de Castile and his wife, who dies due to complications with the labour (which allows Enrique to sidestep the whole bastard deal and just say she's his), manages to convince Louis XI of France, being more compelling than her husband was OTL. With that support, the royal couple successfully convince the Pope to support their claim to the throne over Isabella de Castile's, and with some aid from Spain and the peoples of Castile who recognise Juana as Queen, they manage to take the capital. From there, it seems as God is on her side as she gives birth to an heir in late 1479, and watches her husband take many of the Castilian lands.
After her husband dies in 1481, she finds her stepson less than supportive of her claim, having betrothed his own to the eldest daughter of her aunt, thus ending his support for his brother. Thus, Juana looks elsewhere and in early 1483 marries the King of France's cousin, Count Charles of Angouleme. With his support, they are able to hold onto most of Castile, with the promise of the rest as a dowry when, in 1488, Castile and Aragon agree to peace with a marriage between the Infanta Maria of Aragon and Enrique de Castile and Portugal.
Juana, now officially Queen, finds herself and her son in an odd position when, in 1491, the Prince of Castile's brother, John II of Portugal, is left without his only heir, and thus, his brother, who is also heir to Castile, is left heir to Portugal. And with a wife who had not had a recorded pregnancy since 1482, that meant the potential for the Prince to claim that throne was very high. And, while there was some talk of the King setting aside his wife for a young, presumably more fertile one, in the end he died in 1495 and left Enrique as King of Portugal.
The death of Charles, Count of Angouleme the next year left the Queen of Castile a widow with four of his children: Maria, Charles, Juana and Luisa. Unfortunately, the Princess Juana would suffer an injury in 1499 that left her mentally unstable. However, apart from thus, all five of the Queen's children were healthy and happy, and in the wake of her husband's death she choose to forgo a third marriage, despite offers coming from various noblemen and women.
In 1501 she saw her second son married to Germaine de Foix, a French Noblewoman, as he begins his career as a French nobleman. Her elder son had married his intended, Maria of Aragon, in 1499, despite offers from Ferdinand II of Aragon for him to marry his elder daughter Isabella, who would remain unwed until her death in 1517.
Of the remaining daughters, Maria de Castile would, in 1499, marry James V of Scotland, but died in 1503 after the birth of her first child. Her sister Luisa, meanwhile, married in 1514 the King of France Louis XII, but had no children. She then went on to marry, in 1517, Charles III, Duke of Savoy, giving birth to two sons. Thus, Charles de Castile rose to be Charles IX of France, with his son Henri as his heir.
Now Juan of Aragon, Prince of Girona still dies without issue as OTL, and since Isabella of Aragon won't remarry, guess who now is next in line?!??!?! Maria of Aragon's line inherits Aragon, under her eldest son Juan III de Castile, III de Portugal and III de Aragon. And with that, we have Iberia under one man.
Now no longer the failed Castilian Queen, but the mother to two nations, and a woman who managed to, with a few good matches, unite Iberia and have her second son on the French Throne. There were some bumps along the way, but she has done much better than OTL.
Juana I de Castile (b.1462: d.1530) m. Afonso V de Portugal (b.1432: d.1481) (a), Charles, Count of Angouleme (b.1459: d.1496) (b)
1a) Enrique V de Castile, I de Portugal (b.1479) m. Maria of Aragon (b.1482: d.1517) (a)
1a) Juan III de Castile, Portugal and Aragon
2b) Maria de Castile (b.1485: d.1503) m. James IV of Scotland (b.1473: d.1513) (a)
1a) Margaret Stewart, Princess of Scotland (b.1503: d.1552)
3b) Charles IX of France (b.1488) m. Germaine de Foix (b.1488: d.1530) (a)
1a) Henri, Dauphin of France (b.1510)
2a) Joan of France (b.1511)
3a) Marguerite of France (b.1515)
4b) Juana de Castile (b.1490: d.1509)
5b) Luisa de Castile (b.1493) m. Louis XII of France (b.1462: d.1515) (a), Charles III, Duke of Savoy (b.1485: d.1553) (b)
1a) Charles Emmanuel IV, Duke of Savoy (b.1518)
2a) Victor Philibert of Savoy (b.1522: d.1549)