[1][2][3] Her exact date of birth is unknown, and the year is calculated based on the fact that Matthew Paris reported that the princess got married at the age of 21.

Later, when the problems that accompanied the early years of her brother's reign ended, she was transported to the court, at first located at Woodstock Palace in Oxfordshire and later at Westminster.

[10] The King also celebrated Christmas with Isabella that year; he sent her three of the best dishes from his table and presented many gifts, and then for several months he sent her the items needed to equip the princess's own chapel.

[11] In November, the twice-widowed Frederick II, Holy Roman Emperor at a friendly meeting at Rieti, received the advice of Pope Gregory IX to ask Isabella's hand, and in February 1235 he sent an embassy to King Henry III headed by his chancellor Pietro della Vigna.

[13] After three days of discussion, King Henry III agreed to the marriage;[2] Isabella was brought from her quarters in the Tower to the Palace of Westminster,[14] where she met with the ambassadors, who "declared her the most worthy of the imperial brides", put a wedding ring on her finger and greeted her as their Empress.

[2][15] On 22 February 1235, an agreement was signed,[16] according to which King Henry III provided his sister with a dowry of 30,000 marks (an amount sought by the Emperor in order to fund his wars in northern Italy[17]), which was to be paid within two years, and as a wedding gift he gave her all the necessary utensils, jewelry, horses and rich clothes,[2] all made according to the latest German fashion;[18] also, the princess received patent letters from the Emperor, giving Isabella, as Queen of Sicily and Holy Roman Empress the possession of the lands due to her.

[17] In early May 1235, Archbishop Heinrich I of Cologne and Duke Henry I of Brabant arrived in England to fetch the bride to her new homeland; Isabella departed from London on 7 May under the care and tutelage of the Bishop of Exeter, William Briwere.

[3] Wedding celebrations lasted for four days, and, as they say, they attended "four kings, eleven dukes, counts and margraves, thirty or fewer prelates and minor nobles".

Frederick was delighted with his new wife, but immediately after the wedding he got rid of Isabella's English entourage[2] "of both sexes",[26] leaving only her nurse Margaret Biset and one maid with her, and transferred her to seclusion in Hagenau, where the couple spent most of the winter.

[2][33] In early 1239, Isabella spent some time in Noventa Padovana while her husband was in Padua;[33] in February 1240 she returned to southern Italy, where the emperor soon arrived.

Emperor Frederick II, it seems, respected and loved his wife, but in a quite strange manner:[2] taking care of her safety and surrounding her with luxury and splendor,[32] but keeping at a distance from himself[2] and in company of his "harem", which included women from Arabia;[34] in addition, James Panton writes that the empress was forbidden to communicate with all men, except for the black eunuchs around her.

[38][39] Frederick II at the time of his wife's death was in Faenza,[38][40] and Isabella's dying words were a request to her husband to continue to maintain friendly relations with her brother King Henry III.