On Robert Sewell's site, he cites an article in, The Genealogist, Volume 2, Number 1 (Spring, 1981), which argues that Henry Sinclair, the 2nd Earl of Orkney and Egida Douglas cannot be the parents of Elizabeth (AKA Beatrix) Sinclair, due to consanguinity issues. They propose that Elizabeth Sinclair's father is probably Henry Sinclair, the 1st Earl of Orkney, and that she was his last (possibly even posthumously) born child.
SOME CORRECTIONS TO THE SINCLAIR PEDIGREE
Andrew B. W. MacEwen
Beatrix Sinclair, Countess of Douglas. She was married before 7 March 1425/6 to James Douglas of Balveny, called "the Gross", afterwards 7th Earl of Douglas, and is described on his monument as daughter of Henry; Earl of Orkney. According to The Scots Peerage she was the only daughter of Henry, 2nd Earl of Orkney of that name, by his wife Egidia, daughter of Sir William Douglas of Nithsdale and granddaughter of King Robert II. The Complete Peerage is unsure which Earl Henry was her father.
This is not really a question, however, for the second Earl Henry married a niece of James the Gross. Had Beatrix been a daughter of this marriage, she would have been her husband’s grandniece (of the half blood), since Sir William Douglas of Nithsdale was a natural son of Archibald "the Grim", 3rd Earl of Douglas. Such a relationship is inherently unlikely, and two dispensations disprove it altogether. The sons of Beatrix, William and James, the 8th and 9th Earls of Douglas, were successive husbands of their cousin Margaret, only daughter of Archibald, 5th Earl of Douglas. Her mother, Euphemia Graham, was a descendant of Robert II, while her father the 5th Earl was himself a grandson of Robert III.
The dispensations are dated 24 July 1444 and 27 February 1452/3, and in each the relationship dispensed is the 2nd and 3rd of consanguinity (representing the common descent from Archibald the Grim). Had the Countess Beatrix also been descended both from Robert II and from Archibald the Grim, her sons would have been related to Margaret in three additional ways (4th and 3rd, 4th and 4th, and 4th and 4th of consanguinity), but this is clearly not the case. Beatrix was thus a daughter of the first Earl Henry, slain in 1404, and she was probably his youngest, or even posthumous, child.
Although Countess Beatrix was not her husband’s grandniece, it is perhaps of interest to note that both her brother, the second Earl Henry, and his son, Earl William (of Orkney, afterwards of Caithness), married granddaughters of the same man, Archibald the Grim, Earl of Douglas.
1 She may have been his second wife, unless he remained a bachelor for twenty to twenty-five years after attaining his majority. If he did have an unknown first wife, she was not a daughter of Robert, Duke of Albany.
2 Inscription printed in Sir William Fraser, The Douglas Book, 2 (Edinburgh, 1885) p. 623.
3 6: 570-71.
4 4: 435, n.(c.)
5 The Scots Peerage 3: 163-64.
6 Callendar of Papal Letters 9: 467, 10: 130-31. Communicated by Dr. I Lorne Campbell of London, who had also noticed the problems of Beatrix's parentage.
7 The Scots Peerage 6: 596.