Sources

1) Source: Chancery: Inquisitions Post Mortem, Series I, Edward III "William de Bohun, earl of Northampton...‘Randesbourne’ held for life of the inheritance of Elizabeth his wife, deceased, sister and one of the heirs of Giles de Batelesmere, deceased...Roger de Mortuo Mari, aged 8 years, is son and heir of the aforesaid Elizabeth. Humphrey de Boun, aged 18 years, is her other heir as regards the aforesaid gavelkind lands."Dated: 1360 Eidence for Elizabeth bearing children while with William.. External Link
https://www.british-history.ac.uk/inquis-post-mortem/vol10/pp513-530. Reference: C 135/153/2 (Data from secondary evidence)
2) Source: Testamenta Vetusta: Being Illustrations from Wills, of Manners, Customs Etc. as Well as of the Descents and Possessions of Many Distinguished Families from Henry II. to Queen Elizabeth "May 1356... to Humphrey, my son... to Elizabeth my daughter... to my sister Roos". Reference: Page 60; Elizabeth Countess of Northampton (Data from secondary evidence)
3) Source: Chancery: Inquisitions Post Mortem, Series I, Edward III "Giles de Badelesmere, knight...Elizabeth whom William de Bohun, earl of Northampton, married, Maud whom John de Veer, earl of Oxford, married, Margery whom William de Roos married, [and] Margaret whom John Typetot married, are his sisters and next heirs, and of full age...Elizabeth the wife of William de Bohun, earl of Northampton, aged 28 years, Maud the wife of John de Veer, earl of Oxford, aged 25 years, and Margaret the wife of John Typetot, knight, aged 23...Sellyngge parish. 80a. land, called Rode, similarly acquired by Bartholomew, his father"Dated: 1338. External Link
https://www.british-history.ac.uk/inquis-post-mortem/vol8/pp127-150. Reference: C 135/56/1, C 135/57/1 (Data from secondary evidence)
4) Source: Duchy of Lancaster: Deeds, Series LS "Indenture of marriage articles between William de Bohun, Earl of Northampton, and Richard, Earl of Arundel, relating to Humphrey, son of the Earl of Northampton, and Joan, daughter of the Earl of Arudel, and Richard, son of the Earl of Arundel, and Elizabeth, daughter of the Earl of Northampton."Dated: 1359. External Link
http://discovery.nationalarchives.gov.uk/details/r/C5414206. Reference: DL 27/19 (Data from direct source)
5) Source: Chancery: Inquisitions Post Mortem, Series I, Edward III "William de Bohun, earl of Northampton...‘Randesbourne’ held for life of the inheritance of Elizabeth his wife, deceased, sister and one of the heirs of Giles de Batelesmere, deceased...Roger de Mortuo Mari, aged 8 years, is son and heir of the aforesaid Elizabeth. Humphrey de Boun, aged 18 years, is her other heir as regards the aforesaid gavelkind lands."Dated: 1360. External Link
https://www.british-history.ac.uk/inquis-post-mortem/vol10/pp513-530. Reference: C 135/153/2 (Data from direct source)