This was known in England as the “Sicilian business” or the “matter of Sicily” or similar terms. Edmund had no claim to Sicily, or really anything to do with Sicily at all. It’s pretty weird, and everyone thought so at the time too. How did that happen?

Sicily and England were somewhat related in the sense that they were both ruled by Norman dynasties, although the Norman adventurers who conquered Sicily in the 11th century don’t seem to have been related at all to the royal dynasty of William the Conqueror. By the 12th century, the Norman Kingdom of Sicily encompassed the island of Sicily itself as well as the southern half of the Italian peninsula. The Papal States bordered it to the north, and further north again, on the other side of the Papal States, were quasi-independent city states like Pisa, Florence, Genoa, Milan, etc. - all nominally part of the Holy Roman Empire. (Also Venice, but it was fully independent.)

In 1154, King Roger II died and was succeeded by his eldest surviving son, William I. Roger II had other sons who died before him, including one also named Roger (who died in 1148) - this Roger had an illegitimate son named Tancred. Roger II also left behind a pregnant wife, and his posthumous daughter, Constance, was born later that year. William I died in 1166 and was succeeded by his son William II, who ruled until 1189.

William II was connected to England, as he was married to Joan, the daughter of Henry II of England. But they didn’t have any children, so when William died, Constance was his closest legitimate heir. She had married the Holy Roman Emperor, Henry VI, a few years earlier in 1186. The nobility of Sicily, and the Pope, were worried about the imbalance of power if the Emperor also inherited the Kingdom of Sicily. So in 1189 the kingdom was seized by Tancred.

Tancred also imprisoned Joan. In the meantime, over in the eastern Mediterranean in the crusader states, the Latin Kingdom of Jerusalem had collapsed after Jerusalem was recaptured in 1187. That led to the Third Crusade, one part of which was led by Joan’s brother, Richard I of England. On the way, Richard stopped in Sicily where he attacked Tancred and forced him to release Joan.

Tancred was also challenged by Constance and Henry, since Constance was the rightful heir of the kingdom. Tancred died in 1194 and was succeeded by his son, William III, who was still a child. He was easily overthrown by Constance a few months later. At the end of 1194, Constance gave birth to her son Frederick.

Henry died in 1197 when Frederick was only 3. Once again neither the Sicilian nor German nobles, nor the Pope, wanted the same person to control both the Empire and Sicily, so Constance was forced to give up Frederick’s claim to the Empire. Unfortunately Constance also died the next year in 1198. Frederick was now an orphan, and under the protection of Innocent III, who claimed Sicily as a “papal fief” (i.e. a separate, independent kingdom, but also under Innocent’s protection).

So Frederick grew up as the King of Sicily, and to make a long story short, after a lengthy civil war between different claimants and their factions, the German nobility of the Empire finally decided to elect Frederick emperor after all in 1220. Despite everyone trying to ensure it didn’t happen, in the end Frederick was both emperor and king anyway.

Frederick was extremely ambitious - in addition to surrounding the Papal States, he also married Isabella, the crusader queen of Jerusalem, and claimed to rule the crusader states as well. Isabella died giving birth to their son, Conrad, in 1228. A few years later Frederick married another Isabella, the daughter of King John of England and the brother of King Henry III, so Frederick was connected to England as well.

Frederick promised to go on crusade, but delayed several times (partly because he was afraid the pope would invade his territory while he was gone), and Pope Gregory IX ended up excommunicating him. Frederick finally did go on crusade and managed to regain Jerusalem, through a treaty with the sultan of Egypt - but everyone thought the point of the crusade was to wage war on Egypt, not conclude a friendly treaty with them, so the church still wasn’t happy.

In 1240 Gregory called a council to attempt to depose Frederick, but Frederick attacked the fleet carrying the bishops and imprisoned some of them. Gregory died a few months later and Frederick managed to prevent the election of a new pope for almost 4 years. Gregory’s eventually successor, Innocent IV, did successfully depose him at a council in Lyon in France in 1245. Frederick was still King of Sicily, and still claimed to be Emperor, but as far as everyone else was concerned, there was no emperor.

Innocent IV and various other European states tried to install their own new kings of Germany. Frederick held on for a few years until he died in 1250. After that, although there were numerous claimants, there simply was no emperor at all until 1312.

In Sicily, Frederick was succeeded by his son Conrad, but while Conrad continued to push his own claims to the Empire in Germany, his younger half-brothers Manfred and Henry governed Sicily for him. Henry was the son of Frederick and Isabella of England, while Manfred was illegitimate. Conrad arrived to rule Sicily in person in Sicily in 1252, but he died in 1254. Conrad’s son, Conradin, was still a baby, so Manfred took charge again - although like Frederick had been years before, Conradin was now under the protection of the Pope, who once again claimed Sicily as a papal fief.

Now, at last! This is where Edmund comes in. But we have to go back a little bit earlier first, to 1244. That year, the Christian crusaders in Jerusalem lost the city once again, so a new crusade was organized back in Europe. The crusade was led by Louis IX and it was a huge embarrassing failure, but the French (and the Pope) believed that if Henry III of England had gone too, it would have been a success. Louis was taken prisoner in Egypt during the crusade, and spent a few years in the crusader states after he was released, until he finally returned home in 1254. But in the meantime, the Pope, the French, ambassadors from the crusader states, everyone had been trying to convince Henry to go on crusade too.

Pope Innocent IV tried to entice Henry by getting him involved in the scheme to overthrow Conrad in Sicily - in 1252, Innocent suggested that Henry’s brother Richard of Cornwall should become king of Sicily. Richard wasn’t interested though - supposedly he said that it was as if Innocent was offering him the moon. Innocent instead offered Sicily to Louis IX’s brother, Charles of Anjou, but he didn't want it either (not yet at least). That worried Henry, who didn’t want the French getting any more powerful - they had already conquered Normandy, Anjou, and Poitou during the reign of his father John, and he constantly had to defend the rest of his own territories in France from them. So Henry suggested giving Sicily to his son, Edmund.

This scheme failed for various reasons - Henry wasn’t sure that he should overthrow Conrad, the legitimate ruler of Sicily, or Conrad’s legitimate half-brother, the other Henry, who was after all Henry III’s own nephew. Conrad and his half-brother Henry were both dead by 1254 though, and Henry III had no problem trying to overthrow Manfred, since he was an illegitimate child of Frederick. But the English nobles generally refused to support the “Sicilian business”. They wanted Henry to raise money and men to go on crusade to Jerusalem, but if resources were being diverted to Sicily instead, they couldn’t support that. The rulers of the crusader states were opposed as well, assuming that Henry would get bogged down in Sicily and would never come to help them. Henry himself was also worried about rebellions in England - and indeed the Second Barons' War began about ten years later, partly due to shortsighted plans for far-away adventures like this. So Henry never went on crusade and Edmund never acted on his claim to Sicily.

In 1258, there was a rumour that Conrad’s son Conradin had died - he hadn’t really, but Manfred took the opportunity to declare himself the true king of Sicily, and he was promptly excommunicated by the Pope (at this point the pope was Urban IV). So who was really in charge in Sicily?