Thomas Seymour was born 1508, brother to two other famous Seymours: Jane Seymour, third wife of King Henry VIII and mother of King Edward VI, and Edward Seymour, who became Protector Somerset after Henry VIII's death. Henry VIII had instructed that a Regency Council take charge of the realm during Edward's minority ('Woe to thee O land, when thy King is a child'), but the autocratic and domineering Somerset quickly rose to pre-eminence. As the eldest uncle of Edward this was not without precedent and there would have been little outcry over Edward Seymour's control had he ruled prudently - however he did not, and he himself would be executed on October 14, 1549. But not before he had executed his own brother, Thomas.
Thomas Seymour was the sort of strong, handsome man who was esteemed as a good magnate by his peers because of his strength in tournament and war. But he added to these virtues ambition for power, jealousy and a seditious nature. Henry VIII had given him but a small role in the Regency Council, but with the rise to pre-eminence of his brother this situation couldn't remain. Thomas was made Lord Admiral, Baron of Sudeley and given a place on the Privy Council. But this still doesn't seem to have been enough for him and he was desirous of more power, perhaps out of jealousy for his brother. As a very marriagable man he sought first the affections of Mary Tudor (daughter of Catherine of Aragon and next in line for the throne), but the Council was quick to discourage such a major power grab. Besides, her strict Catholicism barred him from taking her hand. He was likewise headed off wedding Elizabeth Tudor (daughter of Anne Boleyn and second in line for the throne) by the Council, and so he chose to marry Katherine Parr, Henry VIII's widow. She had designs on him before the old King stepped in, and so it was easy for him to persuade her to marry him out of love.
After wedding Katherine in indecent haste and in secret, Thomas then had to exert energy persuading the Council to allow him to woo the woman who was already his wife. Thankfully, they were so relieved that he'd given up on his naked power grabs (for now, at least), that they assented easily. But his marriage to Katherine Parr brought him in close proximity to the original object of his desires, Lady Elizabeth. It was the custom of the time for the households of Ladies to be used as finishing schools for girls, and Elizabeth was resident at Katherine Parr's completing her education. Naturally, Seymour's designs on Elizabeth being well known, this raised eyebrows. Stories then started to filter back from the amiable Seymour household about how Thomas would flirt with the young girl, tickling her in her bed and "striking her on the buttocks familiarly". At first Seymour and his wife were shocked by the rumours circulating, but at length it seems a little jealousy was inspired in Katherine, and she began to accompany her husband to Elizabeth's bedchamber. And so, with Katherine the three would romp and play.
Elizabeth was but a girl of fifteen, but it began to be noticed that she would blush when Seymour were the topic of conversation, and that she began to take pleasure in his affections. In an action which was probably at the instigation of the jealous Katherine, now with child (she died giving birth to it), Elizabeth was sent away to establish her own household. After the death of Katherine, Seymour again turned his desires to Elizabeth, and it seems she was accepting of them: but, as was her character, she was rather too prudent to jump at it. This proved wise, because Thomas was rapidly falling out of favour with the Council, who were now aware of his duplicity over Katherine. The gap between the Seymour brothers was now growing. It had originally been manipulated by the Earl of Warwick, who only days after the death of Henry encouraged Thomas to claim stewardship in equality with Edward, as they were both uncles. Warwick then denounced Thomas to Edward, explaining how he always knew Thomas had been jealous of great power, and that measures should be taken to restrain him! Had the two brothers remained united, Warwick would have found it hard to advance his career: as it was, he became Lord President Northumberland after Somerset's downfall.
Thomas' sedition was growing: he started to try and build a faction around him at Court, gathering followers based on their lands so he could build up a power base. He boasted that as Lord Admiral he had power over a great number of armed men and ships, and appears to have sought control of offshore islands as a path of retreat. He tried to encourage Sir Thomas Wriothesley to join him in a plot against his brother, but instead Wriothesley denounced him to the Council, an act for which he was awarded a place on it! Seymour was sent to the tower on January 17, 1549, amidst rumours that Elizabeth was pregnant by him. Elizabeth was placed under house arrest and questioned extensively by various Royal officals who were convinced she had been complicit in Thomas' plots. They could find no such evidence, but Thomas was attainted for 33 counts of high treason. This smacked of overkill and the people were quick to point out that had Somerset's quarel with his brother being just, the trial would not have been conducted in secret. Thomas Seymour was executed on 20 March 1549, and his death was a direct factor in the fall of Protector Somerset. Despite some power play, if a man would execute his brother, who was truly safe?
Guy, John. Tudor England: Oxford University Press, 1988.
Neale, J. E. Queen Elizabeth I: Penguin Books, 1961.