I think that you are getting a bit mixed up. If you write everything down in date order, it makes sense. A few minutes' research into the inquisitions post mortem and fine rolls gives this pedigree:
1. John de Luccombe (d. bef 26 Oct 1319) = Sibyl (d. bef. 22 Jun 1325)
2. Hugh de Luccombe (1295-Mar 1323) = Elizabeth
3a. John de Luccombe (May 1322-bef. 16 May 1334)
3b. Elizabeth de Luccombe (b.20 May 1320) = Oliver de St. John
26 October 1319, Order to the escheator beyond Trent to take into the king's hand the lands late of John de Luccombe, deceased, tenant in chief.
Calendar of Fine Rolls, vol. 3, Edward II: 1319-1327 (1912), 7.
15 November 1319, Inquisition post mortem of John de Luccombe. Hugh his son, aged 24 and more, is his next heir.
Calendar of Inquisitions Post Mortem, vol. 6, Edward II (1910), No. 235.
http://www.british-history.ac.uk/inquis-post-mortem/vol6/pp139-149
7 December 1319, Order to the same to deliver to Hugh, son and heir of John de Loccecumbe, tenant in chief, the lands late of his said father, he having done homage: saving to Sibyl late the wife of John her dower.
Calendar of Fine Rolls, vol. 3, Edward II: 1319-1327 (1912), 11.
23 March 1323, Order to the escheator on this side Trent to take into the king's hand the lands late of Hugh de Lukcombe, deceased, tenant in chief.
Calendar of Fine Rolls, vol. 3, Edward II: 1319-1327 (1912), 201.
22 May 1325, Grant to Alexander de Sancto Johanne of the county of Devon, for a fine of 100 marks whereof he will pay at the Exchequer a moiety at Michaelmas next and a moiety at Easter following, of the marriage of John, son and heir of Hugh de Luccombe, tenant in chief, a minor in the king's ward; and if the said John die before his coming of age and marriage, his heir being a minor, the said Alexander shall have the marriage of that heir, and so from heir to heir.
Calendar of Fine Rolls, vol. 3, Edward II: 1319-1327 (1912), 347.
22 June 1325, Order to John Everard, escheator in the counties of Cornwall, Devon, Somerset and Dorset, to take into the king's hand the lands which Sibyl late the wife of John de Luccombe, deceased, held in chief in dower of the inheritance of John, son and heir of Hugh de Luccumbe, a minor in the king's ward.
Calendar of Fine Rolls, vol. 3, Edward II: 1319-1327 (1912), 349.
6 July 1325, Order to John Everard, escheator in the counties of Cornwall, Devon, Somerset and Dorset, to take into the king's hand the lands late of John de Luccombe, deceased, tenant in chief.
Calendar of Fine Rolls, vol. 3, Edward II: 1319-1327 (1912), 350.
6 July 1325, John son of Hugh de Luccombe and his wife Elizabeth and heir of his grandfather John de Luccombe was aged 3 years and 8 weeks.
Calendar of Inquisitions Post Mortem, vol. 6, Edward II (1910), no. 627.
http://www.british-history.ac.uk/inquis-post-mortem/vol6/pp393-406
16 May 1334, Order to Ralph de Middelneye, escheator in the counties of Somerset, Dorset, Cornwall and Devon, to make inquisition touching the lands in the king's hand by reason of the minority of John de Luccoumbe, son and heir of Hugh de Luccoumbe, tenant in chief of Edward II, who has died in the king's ward.
Calendar of Fine Rolls, vol. 4, Edward II: 1327-1337 (1913), 402.
24 June 1335, Proof of age of Elizabeth, sister and heir of John, son and heir of Hugh de Luccombe, who died in the King’s wardship. Writ to the escheator to take the proof of age of the said Elizabeth, on the petition of Oliver de Sancto Johanne, her husband, 4 May, 9 Edward III. She was born at Stoklegh Luccombe, co. Devon, on 20 May, 1320, and was baptized on 21 May in the said year by Sir Walter, parish chaplain of the church of All Saints, Cheriton Fitz Payn.
Calendar of Inquisitions Post Mortem, vol. 7, Edward III (1909), no. 697.
http://www.british-history.ac.uk/inquis-post-mortem/vol7/pp479-505
Regards,
John