Edward of Caernarfon's second eldest sister Joan of Acre married Gilbert 'the Red' de Clare, earl of Gloucester and Hertford, on 30 April 1290 when he was forty-six and she eighteen or almost. Their first child, only son and heir Gilbert, earl of Gloucester and Hertford, was born a year later, sometime between 23 April and 11 May 1291, according to the evidence of his parents' Inquisitions Post Mortem taken in January 1296 and May 1307. [CIPM 1291-1300, pp. 234-51; CIPM 1300-07, pp. 311-31]
Gilbert was the eldest grandchild of Edward I and only seven years younger than his uncle Edward II. He married Maud de Burgh, one of the many daughters of the earl of Ulster, on 29 or 30 September 1308 when he was seventeen, and was killed at the battle of Bannockburn on 24 June 1314, aged twenty-three.
Eleanor, Lady Despenser and lady of Glamorgan, eldest of the three daughters of Gilbert the Red and Joan of Acre, was born in October or November 1292. This is according to several books and articles, but I don't know what the source is and can't confirm the date. Eleanor was certainly the eldest of the three de Clare sisters, and the date of birth appears to be plausible. She married Hugh Despenser the Younger at Westminster on 26 May 1306 when she was probably thirteen and a half. Eleanor died, according to her (incomplete) IPM, on 30 June 1337 at the age of forty-four, leaving her eldest son Hugh or Huchon Despenser as her heir; he was then said to be between twenty-six and twenty-nine years old. [CIPM 1336-46, pp. 78-9
The date of birth of the second sister Margaret de Clare, countess of Cornwall and Gloucester, is not known. Her Wikipedia page gives 12 October 1293 as the date, but I haven't the faintest idea where that comes from. Who knows where Wikipedia contributors find stuff? That date strikes me as a little too close to the date of birth of Margaret's sister Eleanor in October/November 1292 to be plausible, and a little too far away from her younger sister Elizabeth's birth in September 1295. Gilbert 'the Red' and Joan of Acre were in Ireland from June 1293 to April 1294, so Margaret may have been born there, or not long after they returned to England. A date of birth in the spring or early summer of 1294 would give a more regular spacing between Margaret and her siblings and would make her about thirteen and a half when she married Piers Gaveston on 1 November 1307; her sisters were also both thirteen when they married. She married her second husband Hugh Audley on 28 April 1317.
Wiki gives 9 April 1342 as the date of Margaret's death, as do many other websites. Her Inquisition Post Mortem in fact says that she died on ‘Tuesday the morrow of the Close of Easter last’ in Edward III's sixteenth regnal year, which ran from 25 January 1342 to 24 January 1343. Easter Sunday in 1342 fell on 31 March, so Margaret therefore would seem to have died on 2 April 1342, not 9 April. [CIPM 1336-46, pp. 253-5] Her heir was her only surviving daughter, Margaret Stafford née Audley, from her second marriage to Hugh Audley. Margaret Stafford was said to be either eighteen or twenty years old at the time of her mother's death, which places her date of birth around 1320 or 1322.
The youngest de Clare sibling, Elizabeth, was supposedly born on 16 September 1295, just a few weeks before their father Gilbert 'the Red' died in early December 1295. I'm not sure what the source for her date of birth is; I'm not saying it's incorrect, just that I haven't, yet, seen the document which gives it. Elizabeth married 1) John de Burgh, son and heir of the earl of Ulster, on 30 September 1308; 2) Theobald de Verdon, around early February 1316; and 3) Roger Damory, shortly before 3 May 1317. She had one child from each marriage. Elizabeth died on 4 November 1360, aged sixty-five, leaving her granddaughter Elizabeth de Burgh, countess of Ulster, as her heir. The younger Elizabeth was born in 1332 as the only child of the elder Elizabeth's son William Donn de Burgh, earl of Ulster (1312-33), and married Edward III's second son Lionel (b. 1338). [CIPM 1352-60, 507-13]