Mary of Scotland, Countess of Angus - Where is Matilda Edmonstone?

@ Mary Edmonstone

Is currently shown as the Daughter of: Mary of Scotland, Countess of Angus and her Fourth Spouse: William Edmonstone, Lord Edmonstone of Culloden

I'm not so sure about the above: 'Lady Elizabeth (or Mary) Edmonstone (listed first)' as being a Daughter of Mary (listed second) and William (listed third).

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A system of heraldry, speculative and practical, with the true art of blazon, according to the most approved heralds in Europe: illustrated with suitable examples of armoria figures, and achievements of the most considerable surnames and families in Scotland, together with historical and genealogical memorials relative thereto
by Nisbet, Alexander, 1657-1725 ~ Publication date 1816 ~ Volume 2, Appendix By: Robert Fleming.

https://archive.org/details/systemofheraldry02nisbuoft/page/156/mod... (see: bottom of page 157)

"This Sir William Edmonstone of Culloden died in the year 1462 (n), and left issue bv Mary Countess of Angus, his wife aforesaid, only one son, William, who was his father's heir and successor in his estate, and a daughter, Matilda, who was married to Sir Adam Cunningham of Capringtoh, in the shire of Ayr, and had issue (o)."

Where is Matilda Edmonstone? She is not connected to her parents in Geni.com as showing to be Mary and William's Daughter in the above link.
Why is William's Death Date in Geni.com: 'before 1460' and not 1462 as also shown in the above link?

Thank You, John