Three Sisters, Three Queens by Phillippa Gregory, 2016

Yet another 16th Century royal page-turner, Three Sisters is told from the viewpoint of Margaret Tudor, Henvy VIII's older sister.  Filled with political maneuverings, wars and constant plotting for the gain of power, the Tudors were not a boring family.

Betrothed to James II, King of Scotland, Margaret's union establishes peace along the English-Scottish border.  At the Tudor court, Katharine of Aragon lives in penury as the widow of Arthur.  Her father refuses to support her until Arthur's father, Henry VII pays her dowry.

With Henry VII's death, Margaret's brother Henry VIII becomes the King of England and marries Katharine of Aragon, making her Queen.  Katharine is restored to her proper place in the royal court & the two queens now focus on producing an heir for their respective thrones.

During Henry's reign, he attacks France, who has a treaty with Scotland, thus going to war with Scotland.  During Henry's siege of France, James II sees an opportunity to claim English lands given the lack of men remaining in England.

With Katharine Queen Regent during Henry's absence, she leads a decisive and brutal victory over James and his men.  She even orders the head of James, her brother-in-law, leaving Margaret a widow.

Margaret is seduced by a Scottish lord, Archibald Douglas, and marries him without approval from the Scottish lords.  Margaret writes to her brother & Katharine to send men and reinforcements, but none are provided. 

With no defense against the Scottish lords who turn on her, she is forced to flee to England.  She returns to Scotland to be with her son, but she and her son remain siege under the Douglas clan.  The Scottish lords who do rise to defend her are slaughtered by the Douglas clan.

Henry's and Margaret's younger sister is a renowned beauty betrothed to Charles of Castille, the grandson of Louis XII, King of France.  Before Charles becomes of age, his grandfather becomes a window and sends his men to the Scottish court to discuss a possible union between Scotland and France.

As Margaret considers the proposal and the powers he would have, she discovers that her brother Henry has been making arrangements and betroths Mary to the much older Louis XII to unite England and France.

With Mary's marriage, she becomes Queen of France.  The three sisters had all successfully ascended the throne, a remarkable political feat.