Edward de Monthermer died in late 1339 or early 1340; I haven't found a reliable source for the date of his death, although Jennifer C. Ward says he was mortally wounded at the battle of Vironfosse in October 1339 and died in early December, and Edward III ordered "the lands late of Edward de Monte Hermerii, deceased, tenant in chief" to be taken into his hands on 3 February 1340.  Edward died in debt: John de Holdich, one of his executors, begged Edward III for money the king owed to Edward, as Edward "owed many men money and this remains unpaid, and Holdich has suffered great problems in the burial of Mounthermer, and he has not been able to perform the will without payment or assignment."  Either because of the executors' financial problems or out of familial affection, or both, Edward's half-sister Elizabeth de Clare arranged and paid for his funeral, burying him next to their mother Joan of Acre at the Austin friary in Clare, Suffolk. There is evidence of a close relationship between Elizabeth and Edward; she bought him a palfrey in 1338, for example, and he appears to have been living in her household at the time of his death. 
Edward de Monthermer was in his mid-thirties when he died, unmarried and childless. His elder brother didn't outlive him for very long: Thomas was killed at Edward III's great naval victory over Philip VI of France at Sluys on 24 June 1340. He had married, in 1327 or thereabouts, a woman named Margaret, who is thought to have been the widow of Sir Henry Tyes, a Contrariant executed by Edward II in March 1322. (Douglas Richardson thinks Margaret was the daughter of one Piers de Braose.) Although Edward II is often criticised, not least by me, for his vindictive and unpleasant treatment of the wives and children of the Contrariants, he gave Margaret and her late husband's sister Alice, whose husband Warin Lisle was also executed, a generous allowance of 200 pounds a year on 6 April 1322, two weeks after their husbands' executions, and they weren't imprisoned. 
Thomas de Monthermer and Margaret Tyes had one child, Margaret de Monthermer, born on 14 October 1329 and one of only two grandchildren of Ralph de Monthermer and Joan of Acre (the other being Isabella MacDuff, countess of Fife), and the heir of her father and grandfather. She married John Montacute or Montague, second son of Edward III's close friend William, earl of Salisbury (died 1344) and Katherine Grandisson. Margaret de Monthermer and John Montacute had one son, also John, born around 1350, who succeeded his uncle William as earl of Salisbury in 1397 and was beheaded in January 1400 following the failure of the Epiphany plot to restore the deposed Richard II to the throne. Through the Montacute line, Thomas de Monthermer was the great-great-great-grandfather of Richard Nevill, earl of Warwick (1428-1471, the Kingmaker), and was also the ancestor of Queens Anne Boleyn and Katherine Parr.
10) Close Rolls 1327-1330, p. 530; Patent Rolls 1327-1330, p. 547.
11) E.M. Thompson, ed., Adae Murimuth Continuatio Chronicarum, p. 256.
12) Close Rolls 1330-1333, p. 14.
13) Close Rolls 1330-1333, p. 74; Patent Rolls 1330-1334, p. 33.
14) Jennifer Ward, Women of the English Nobility and Gentry 1066-1500, p. 81; Fine Rolls 1337-1347, p. 158.
15) The National Archives SC 8/177/8809.
16) Ward, Women of the English Nobility, p. 81; Underhill, For Her Good Estate, p. 88.
17) Close Rolls 1318-1323, p. 666.