Isabel of Castile was born sometime in the year 1355 in Tordesillas in central Spain, probably in the royal monastery of Santa Clara, where her father and her grandfather Alfonso XI (b. 1311, r. 1312-50) had built a palace. Isabel was the third daughter of Pedro 'the Cruel', king of Castile and Leon (b. 1334, r. 1350-69) and his long-term mistress Doña María de Padilla, a Castilian noblewoman; her older sisters were Beatriz, born in Córdoba in southern Spain in 1353, who died sometime after 23 September 1366*, and Constanza, born in Castrojeriz in northern Spain in June or July 1354, later duchess of Lancaster. The sisters had a younger full brother, Alfonso, but he died at the age of three in 1362 in the city of Seville, and they had at least five other illegitimate half-siblings, born to Pedro's other lovers. The Cortes of Castile declared the three sisters legitimate after King Pedro pretended that he had married María de Padilla in 1352, the year before he wed his unfortunate French wife Blanche de Bourbon, whom he imprisoned shortly after their wedding and who died still in prison in 1361. Constanza, as the elder of the two surviving royal Castilian daughters, inherited her father's kingdoms, at least nominally, after Pedro was killed by his illegitimate half-brother and deadly enemy Enrique of Trastámara in March 1369. In reality, Trastámara became King Enrique II of Castile and Leon, and was succeeded by his son Juan I in 1379 and then his grandson Enrique III in 1390.
* When King Pedro talked of "our three daughters" (tres filiae nostrae): Foedera 1361-77, pp. 805-6.
Constanza and Isabel of Castile moved to England in the early 1370s and married the royal brothers John of Gaunt, duke of Lancaster and earl of Leicester, Lincoln, Derby and Richmond, and Edmund of Langley, earl of Cambridge, sons of Edward III and Philippa of Hainault. If their elder sister Beatriz had still been alive at this point, she would have been their father's rightful heir, and John of Gaunt, who named himself king of Castile and Leon from 1372 onwards, would have married her, rather than Constanza. Edmund of Langley, born c. 5 June 1341, was fourteen years older than his new wife, and was thirty-one to her seventeen, or sixteen going on seventeen, when they married in July 1372. Isabel became a duchess in 1385 when Richard II granted his uncle Edmund the dukedom of York, and had three children: Edward, duke of York, killed at the battle of Agincourt in 1415; Constance, Lady Despenser and briefly countess of Gloucester (d. 1416); and Richard, earl of Cambridge (executed 1415), grandfather of Edward IV and Richard III. Duchess Isabel died at the age of only thirty-seven on 23 December 1392, leaving a will which she made in French a few weeks before her death, now held in the National Archives.
A translation of Isabel of Castile's will was printed in 1826 in the first volume of Testamenta Vetusta, but unfortunately it is much shortened and has led numerous writers, including myself, astray regarding the nature of Isabel's relationship with her husband. Isabel's entry in the Oxford Dictionary of National Biography states, entirely wrongly but understandably, that she made no legacies to her husband and that her will consists of "a few bequests of jewellery". Nope, really, really not. Having seen and read the entire will, it is astonishing to me just how abbreviated the Vetusta translation is; it's maybe 5% of the original text. The sub-title of Testamenta Vetusta does say Being Illustrations from Wills, and it doesn't claim to translate all the text of the fourteenth- and fifteenth-century wills it includes, but still, the translations are highly misleading because it isn't made clear that the editor translated every fifth or tenth or twentieth line of each will and left out the rest. Its 'illustration' of the 1380 will of Edmund Mortimer (1352-81), third earl of March, for example, takes up just two and a half pages of Vetusta, but the full will, printed in the original French in A Collection of All the Wills Now Known to be Extant of the Kings and Queens of England, Princes and Princesses of Wales, and Every Branch of the Blood Royal, runs to thirteen pages. John of Gaunt's will is twenty-eight pages in A Collection, but only five pages in Testamenta Vetusta; Edmund of Langley's will is less than a page in Vetusta and two and a half pages in A Collection. So I advise considerable caution if you're using Testamenta Vetusta as a source.
That being the case, here's some more information about Isabel of Castile's will of 1392 and what she actually bequeathed, not the absurdly abbreviated and hopelessly confusing version of it in Vetusta. The duchess left items to the following people, named here in the order in which they appear in the will (in order of rank, according to the strict hierarchical etiquette of the era): her nephew-in-law Richard II, king of England; Richard's queen, Anne of Bohemia; Isabel's brother-in-law John of Gaunt, duke of Lancaster; her husband Edmund of Langley, duke of York; all three of Isabel and Edmund's children, Edward, Constance and Richard; Eleanor de Bohun, duchess of Gloucester and the wife of Thomas of Woodstock, Edmund of Langley and John of Gaunt's youngest brother; and, rather intriguingly, John Holland, earl of Huntingdon. Isabel also left numerous items and gifts of cash to a long list of her servants and attendants, none of which are mentioned at all in the Vetusta abstract of her will, so you'd have no idea of her generosity and kindness to them unless you make the effort to locate and look at the original text of her will.
John Holland, earl of Huntingdon, was the only noble/royal person in the will who wasn't a close relative of the duchess of York by blood or marriage, though he was Richard II's half-brother and was married to John of Gaunt's daughter Elizabeth of Lancaster. It's also, to my mind, rather fascinating that Isabel left nothing at all to her older sister Constanza of Castile, duchess of Lancaster, and failed even to mention her, and although she left items to Eleanor, duchess of Gloucester, she gave nothing to Eleanor's husband Thomas of Woodstock, her (Isabel's) own brother-in-law.
To John Holland (al counte de Huntyngdon), Isabel left her two Bibles, and "the best fillet that I have". The meaning of 'fillet' in this context is unclear to me; it usually meant a decorative headband of interlaced wire or a string of jewels to be worn on the head, but this was not an item one would expect to see being bequeathed to a man. There's definitely no squiggle above the word counte, 'earl', however, to indicate that the word is abbreviated and that Isabel actually intended to refer to the countess, Elizabeth of Lancaster, and the al, 'to the [earl]', is in the masculine form anyway. As well as these items, Isabel left to her elder son Edward some valuable gifts which John Holland had given to her: a gold cup, a gold brooch with "very large pearls and three sapphires" and a gold chaplet with white flowers, all of which "the earl of Huntingdon gave me" (q' le counte de Huntyndon me donna). The gold cup Holland had given her was engraved with Isabel's arms - presumably she meant the arms of Castile - so evidently he had had it especially made for her. Given the theory that Isabel of Castile and John Holland had an affair, this does perhaps add some fuel to that particular fire. Although the English nobility did send each other gifts on a regular basis, the presents John Holland gave to her were the only ones Isabel mentioned in her will.
To Anne of Bohemia, "my very dread lady the queen" (ma tresredoutee dame la Royne), Isabel gave seven gold belts; to Anne's husband Richard II, she gave a drinking-horn studded with pearls; and to the man who was both her husband's brother and her sister's husband, John of Gaunt, whom she called "my very honoured brother of Lancaster", Isabel gave a tablet of jasper which "the king of Armonie gave me". That's a reference to Levon or Leo, king of Cilician Armenia, who had visited England some years before. Duchess Isabel also left several tablets of gold and a "psalter with the arms of Northampton" to "my very honoured sister of Gloucester", i.e. Eleanor de Bohun. The duchess's father Humphrey de Bohun (1342-73) had been earl of Northampton, which suggests that the psalter had been a gift to Isabel from a member of the de Bohun family in the first place (I might speculate that it was a wedding gift to Isabel from Humphrey in July 1372, a few months before he died, but obviously I can't know for sure).
Isabel referred to Edmund of Langley as her "very honoured lord and husband of York", and left him all her horses, all her beds (tous mez litz) including the cushions, bedspreads, canopies and everything else that went with them, her best brooch, her best gold cup, and her "large primer". It is emphatically not the case that she did not bequeath him anything, and why the translator/editor of Testamenta Vetusta left all of this out is beyond me (he also left out her bequests to John Holland). To her elder son Edward, earl of Rutland (counte de Ruttellond), Isabel left the items mentioned above which she had been given by John Holland, and her crown, leaving him strict instructions never to sell it or give it away but to keep it in the family. To "my beloved daughter [ma t'samee fille] Constance la Despenser", Isabel gave a fret, i.e. a head-covering of interlaced wire, with pearls, her best fillet, and her silver scelle, which could mean saddle or stool. She then asked her nephew-in-law the king to "take his humble godson [humble filiol] to heart", meaning her youngest child Richard of Conisbrough, who was a good bit younger than his two siblings and wasn't specifically left any bequests except his mother's "beautiful psalter".
Isabel also left items and gifts of money to numerous servants of hers, both men and women; just to pick out a couple of examples more or less at random, she gave thirty shillings to 'John of the Wardrobe', ten marks to Roger Palfrayman, and good brooches to Elianor Southfeld and Marie Weston. Another female attendant of Isabel's named in the will, to whom she left a gown and a cloak, is a woman stated by the chronicler Jean Froissart to have been the mistress of the duchess's brother-in-law John of Gaunt; more info is available in my biography of him, due out early next year.