Who were the parents og Margaret de Holkettle, Heiress of Holkettle ?

Margaret, heiress of Holkettle

===Who were the parents og Margaret Heiress of Holkettle ?===

Alexander de Betune was killed in the great battle of Duplin, anno 1332.

His son, Robert de Betune married Janet Balfour, daughter of Michael de Balfour, of that Ilk, and sister to John Balfour.
Robert died 1346 without children.
Roberts estate fell to John Bethun, his nephew, the son of Robert Bethun and Janet Balfour.

This John, first of the Bethuns of Balfour, acquired from Duncan, Earl of Fyffe (Probably Donnchadh IV, Earl of Fife), the lands of Holkettle.
The same John married Catherine Stewart, daugther of Margaret Heiress of Holkettle and Robert Stewart Baron Lorn of Innermeath.

Catherine Stewart was born abt 1346 and Duncan, Earl of Fyffe died in 1353. But could it be that her mother Margaret died before Duncan, so that the title "Heiress of Holkettle" was from Duncan, Earl of Fyffe as her father ???

If that was the case, John acquired the lands of Holkettle from his father in-law. Cathrine also has the title "Heiress of Holkettle".

Source :
*http://www.archive.org/stream/miscellaneouspa02unkngoog/miscellaneo... (search for Holkettle)
*http://wc.rootsweb.ancestry.com/cgi-bin/igm.cgi?op=GET&db=jwebe...

Can anyone confirm or deny my theory? I do not know any of the original sources of the information I have found. It is of course very possible that my proposal is completely wrong.