John Forbes, 6th Lord Forbes was born before 1477.2 He was the son of William Forbes, 3rd Lord Forbes and Lady Christian Gordon.3

He married, firstly, married Catherine Stewart, daughter of John Stewart, 1st Earl of Atholl and his 1st wife, Margaret Douglas. Their children included:

1. Elizabeth Forbes, who married John Grant of Freuchie (died 1585)
2. James Forbes (died young)

He married, secondly, Christian Lundie, daughter of Sir John Lundie of that Ilk and Elizabeth Forrester, before 26 February 1509/10.3

Brad Verity wrote:[1]

She is called second daughter of the 1st wife in DP and CP sub Forbes (1926), 1st daughter of the 2nd wife by BEP, and listed as the 2nd daughter in SP Atholl (1904).

No marriage contract or settlement survives to verify the first name and marriage of this daughter of Atholl. We do have this, from a genealogy of the Forbes family written by one Matthew Lumsden, around the year 1580, when he died:

"Next to him succeeded John Lord Forbes, the third brother, gray Willie's youngest son, who married Dame Katharine Stewart, daughter to Earle John of Atholl, who bear to him James Master of Forbes, who died unmarried, whilk was a great pitie and dolor to them yt. knew him; and a daughter called Elspet, who was married on the Laird of Grant, and his Ladie departed but further succession: and after her death, the sd. Lord John married Dame Christian Lundie, daughter to Sr John Lundie of that ilk"

https://archive.org/stream/genealogyoffamil00lums#page/10/mode/2up

As Matthew Lumsden, the author of the above, was married to an illegitimate daughter of John, 6th Lord Forbes, presumably he was familiar enough with the family to correctly report the first name and parentage of the 6th Lord's Stewart wife.

Katherine Stewart died before 26 February 1510, when King James IV approved Forbes's settlement on his second wife Christine Lundy:

https://archive.org/stream/registrummagnisi02scot#page/734/mode/2up

Katherine Stewart's surviving child Elizabeth Forbes married James Grant, 3rd Laird of Freuchie, and three of their children were married in 1539, 1545, and 1553, pointing to births for them in the 1520s. Chronology then suggests that Elizabeth Forbes was born around 1500, and that in turn her mother Katherine Stewart fits better as a daughter of the 1st Earl of Atholl and his second wife.

This is only guesswork. The 6th Lord Forbes doesn't have an entry in ODNB and only has a barebones account in CP, making it difficult to determine any links with the Stewarts of Atholl that could help to date his first marriage.

Origins

John Stewart and Margaret Douglas had three daughters:

1. Lady Elizabeth Stewart, of Atholl, married Andrew Gray, 2nd Lord Gray.
2. Jean (or Janet) Stewart, married by contract dated 31 A |ugust 1507, James Arbuthnott of Arbuthnott. He had a crown charter of the feudal barony of Arbuthnott 29 January 1507. He left a testament dated 7 March 1521, and died before 13 March 1521. [SIC: daughter by 1st wife, Margaret Douglas]
3. Katherine Stewart, married Sir John Forbes, 6th Lord Forbes. [SIC: daughter by 1st wife, Margaret Douglas]

John Stewart and Eleanor Sinclair had two sons and nine daughters:

1. John Stewart, 2nd Earl of Atholl, living 1520, and died before 1522, who married Janet Campbell (died about Candlemas 1545/6), daughter of Archibald Campbell, 2nd Earl of Arygll.
2. Andrew Stewart, Bishop of Caithness.
3. Elizabeth | Isabel Stewart, married John Stewart, 3rd Earl of Lennox
4. Lady Christian Stewart, married Niel Stewart, 3rd of Garth and Fothergill d. 30 May 1538
5. Margaret Stewart, married Sir William Murray, of Castleton who was killed at Battle of Flodden Duke of Atholl
6. Elizabeth | Isabel Stewart, married Alexander Robertson, of Strowan.
7. Jean Stewart, married Robert Gordon, Knt., of Pitlurg.
8. Isabel | Margaret Stewart, married Robert Robertson younger, of Strowan.
9. Marjory [sic: MARIOTE] Stewart, married Sir Colin Campbell, of Glenorchy. d. 26 Jul 1529, Lochtay
10. Elspeth Stewart, married Sir Robert Innes 2nd of Innermarky.

From: [http://en.wikipedia.org/wiki/John_Stewart,_1st_Earl_of_Atholl]

Katherine Stewart Born about 1473 in Of, Aberdeenshire, Scotland Daughter of John Stewart and Margaret (Douglas) Stewart Sister of Jean (Stewart) Arbuthnot [half], Elizabeth (Stewart) Gray, Joanna (Stewart) Gordon, Christian Stewart [half], Elspeth (Stewart) Innes [half], Isabel (Stewart) Robertson [half], Elizabeth (Stewart) Ross [half], John Stewart [half], Elizabeth (Stewart) Robertson [half], Jean Stewart [half], Andrew Stewart [half], Margaret Stewart [half] and Mariote (Stewart) Campbell [half] Wife of John Forbes — married before 1508 in Scotland Mother of Elizabeth (Forbes) Grant and James Forbes Died about 1515 in Scotland

1463 Traquair, Peebleshire, Scotland Death: February 26, 1510 (47) Scotland Immediate Family:

Daughter of Sir John Stewart, 1st Earl of Atholl and Lady Margaret Douglas, Countess of Douglas Wife of John Forbes, 6th Lord Forbes Mother of James Forbes and Hon. Elizabeth Forbes Sister of Isabella Stewart; Jean Stewart, Countess of Huntly; Elizabeth Stewart and Anne Stewart Half sister of Marjory Stewart; Margaret Stewart; Elspeth Stewart; John Stewart, 2nd Earl of Atholl; Christian Stewart; Lady Elizabeth Stewart; Jean Stewart; Andrew Stewart, Bishop of Caithness; Jean Stewart; Elizabeth Stewart; Henry Stewart; Malcolm Leslie, First Laird of Cults and Andrew Leslie of Pitscurry

Sources

1. http://archiver.rootsweb.ancestry.com/th/read/GEN-MEDIEVAL/2014-05/...
2. Source: Sir James Balfour Paul, The Scots Peerage: founded on Wood's Edition of Sir Robert Douglas's Peerage of Scotland. (Edinburgh: David Douglas, 1908-1919. 9 vols.), IV:54. (No entry in Vol. 9, corrections)
3. Source: G E C et al, Complete Peerage (St Catharine press, London, 1910 on), V: 545 (Forbes). (No entry in Vol XIV, corrections)
4. John Stewart, 1st Earl of Atholl (c. 1437–1442 – 15 September 1512),[1] also known as Sir John Stewart of Balveny, was a Scottish nobleman and ambassador. Sometime before April 1475, he married as his second wife, Lady Eleanor Sinclair (died 21 March 1518), daughter of William Sinclair, 3rd Earl of Orkney and Marjory Sutherland. They had two sons and nine daughters: ... Lady Catherine Stewart, married John Forbes, 6th Lord Forbes.
5. John Forbes, 6th Lord Forbes (died 1547) was a Scottish landowner. He was the son of William Forbes, 3rd Lord Forbes and Christian Gordon, daughter of Alexander Gordon, 1st Earl of Huntly. John Forbes married Catherine Stewart, daughter of John Stewart, 1st Earl of Atholl and Eleanor Sinclair. Their children included: Elizabeth Forbes, who married John Grant of Freuchie (died 1585). John Forbes married secondly Christian Lundie or Lundin.
6. en.Wikipedia: John Forbes, 6th Lord Forbes
7. Montgomery Rowntree Families and Genealogy: 6th Lord Forbes John Forbes
8. Matthew Lumsden, Genealogy of the Family of Forbes (Inverness, 1819), p. 12. < GoogleBooks >
9. James Balfour Paul, Scots Peerage, vol. 4 (Edinburgh, 1907), p. 53. < Archive.Org >
10. "Red Book of Scotland, Volume 9" (Smi-You). Pages 580-582. < Ancestry.com >; (Document attached). Sir John Stewart, 1st Earl of Atholl, m. firstly, by 25 March 1460, to Margaret, daughter of Archibald Douglas, 5th Earl of Douglas, by whom he was the father of: Janet, married Alexander Gordon; Catherine, married John, 6th Lord Forbes; Elizabeth Stewart, married before 1483, to Andrew, 2nd Lord Gray, (he was widower of Janet, only daughter of Robert, Lord Keith, by whom he had issue. he d. in February of 1514), and had issue. He married secondly, by 19 April 1475, to Eleanora, daughter of William Sinclair, Earl of Orkney and Caithness (she d. on 21 March 1518) .... and by whom he had further issue [2 sons and 9 daughters].
11. https://genealogics.org/getperson.php?personID=I00006030&tree=LEO cites
    1. [S00058] Doubleday, H.A. & Lord Howard de Walden, The Complete Peerage, 1936 .5:545
    2. [S01352] ~Burke's Peerage and Baronetage 1938 . 1029
    3. [S00120] Dupont, Jacques and Saillot, Jacques, Cahiers de Saint Louis . 865
    4. [S00010] Burke, Sir Bernard, A Genealogical History of the dormant, abeyant, forfeited and extinct peerages of the British Empire, London, 1866. 509
    5. [S01534] Watney, Vernon James, The Wallop Family and Their Ancestry 1928 . 729