She’s placed as daughter of Sir Geoffrey de DINHAM Knight (d 26 Dec 1258 in Buckland Dinham, Somerset, England) by genealogist Hal Bradley:
https://freepages.rootsweb.com/~hwbradley/genealogy/aqwg1357.htm#22361
https://freepages.rootsweb.com/~hwbradley/genealogy/aqwg1800.htm#32175
citing:
https://freepages.rootsweb.com/~hwbradley/genealogy/aqwc1800.htm#32...
1Maclean, John, The Parochial and Family History of the Deanery of Trigg Minor, in the County of Cornwall (London: Nichols & Son, 1873-1879.), 3:158, Family History Library, 942.37 K2ma.
2Vivian, John Lambrick, The Visitations of Cornwall: Comprising the Heralds' Visitations of 1530, 1573 & 1620 (Exeter: Pollard, 1887.), p. 72, Los Angeles Public Library, Gen 942.37 V858.
3The National Archives, AR/35/1 - 2.
Medievalist Ronny Bodine at soc.Gen.medieval has her a generation later, as daughter of Geoffrey’s son Oliver (d 26 Feb 1299) and sister of Josce/Joyce de Dinham (died 30 Mar 1301).
https://groups.google.com/g/soc.genealogy.medieval/c/5_L4n1kMODc/m/...
From the foregoing and the apparent chronology provided therein, and by Ronny Bodine in prior SGM posts [2], it would appear most likely that Joan, wife of Roger de Carminow and mother of (A) Oliver de Carminow and (B) Joan de Carminow, wife of William de Whalesborough, was a sibling of Sir Josce de Dinham (fl. ca. 1273-1300) and a daughter of Sir Oliver de Dinham of Hartland, Nutwell and Ilsington, Devon, Buckland Denham, Somerset and
Cardinham, Cornwall (d. 26 Feb 1298/9). This would add a bit to the ancestry of the Carminow and Whalesborough families.
More quotes from SGM in the Wikitree profile for her:
https://www.wikitree.com/wiki/Dinham-6#Evidence_that_Joan_was_a_Dinham
Evidence that Joan was a Dinham
In 1309 Joan, widow of Roger Carminow, made an agreement about her dower with her eldest son Oliver. The document has a seal with the arms of Carminow and Dinham together: "The arms on the shield suggest that Joan was a Dinham by birth, which seems not to be otherwise known (though it is asserted, without authority, by J.P. Yeatman, The Early Genealogical History of the House of Arundell, p. 257b)." Quoted here, which goes on to say: "Oliver de Dinham held the lands of Roger de Carminow senior and the wardship of his heir in 1277, according to James Whetter's "Cornwall in the Thirteenth Century", page 151. Whetter goes on to name Joan, wife of Roger junior, as being a Dinham."
It appears that Oliver Dinham, guardian of the underage Roger Carminow and Roger's inheritance, arranged Roger's marriage to his daughter Joan, whose eldest son was named Oliver.
Year of Birth
Joan's eldest son Oliver was "described as being thirty years of age at his father's death" in 1308.[1] If Oliver was born around 1278, this suggests a birth year for Joan of around 1260. This fits the assumption that her presumed father, Oliver Dinham, as guardian of the under-age Roger Carminow in 1277 (see below), arranged for Roger's marriage to his daughter Joan. It also means that Joan was the daughter of an unknown first wife of Oliver Dinham, as he didn't marry Isabella de Vere until about 1275.
Wikitree also cites Extinct Cornish Families, Part II by Mr. W.C. Wade, who has no origins for her:
https://patp.us/reading/extinct-cornish-families
The tomb at Mawgan-in-Menage is that of Sir Roger Carminow, the most distinguished member of his family. He was the grandson of Robert Carminow, of whom the first regular record is traceable in Col. Vivian and Dr. Drake's Cornish pedigrees. The surname of his wife, whose figure lies beside his, is lost; but her Christian name was Joanna, which seemed afterwards to have become a favourite one in this family.