It made little difference for Seymour when his sister died within weeks of the prince's birth, and he continued to serve Henry VIII, taking on more political posts throughout England and Scotland in the years to come. One of his most prominent roles during this time was as lieutenant-general of the north, when he was instructed to 'punish' the Scots for their rejection of the Treaty of Greenwich and a broken betrothal plan between the young Prince Edward and the infant Mary, Queen of Scots. Starting around 1544, this strife in Scotland would continue for years (known as 'the rough wooing'), causing much financial hardship for England - not to mention many casualties for Scottish soldiers.
But Edward's real rise came upon the death of Henry VIII in 1547, when the nine-year-old Edward VI was proclaimed king, but obviously unable to rule in his own right, due to his age. In Henry's will, it stipulated that a Council of sixteen trusted men would be formed to govern England and guide young Edward until his eighteenth birthday - at which time he could finally be in charge. It is important to note here that Henry VIII had explicitly determined that it was safest for the realm if all governmental power was divided among these men, and that no single nobleman should be named 'leader'. Despite this, the Council decided soon after that Seymour should be named 'Lord Protector', and he quickly capitalized on this new role by awarding himself and others with titles, lands, and riches (in short, he became even more popular among men of the Council, because he was already taking care of them).
In a desperate attempt to maintain control, Seymour took the king (age twelve at the time) to Windsor Castle on October 7th, 1549 - for reasons that aren't all that clear. Obviously, it appeared malicious to the rest of the Council, because Seymour was arrested on October 11th - while Edward VI was restored to safety under the rising leader of the Council, John Dudley, Duke of Northumberland.
Edward Seymour's reputation is an interesting one. While some historians maintain that he was nothing more than a power-hungry, 'evil' nobleman who attempted to manipulate Edward VI at all costs for his own gain, others have insisted that his intentions might have been slightly more noble - that he attempted to solidify the young king's government and unify it under one clear religion. While making some bad financial and political decisions, he is still generally seen as 'less evil' than his counterpart, John Dudley, who is notoriously called 'the bad duke'.