Her birth date is supposedly before 1380. Read the profile notes for discussion about her parentage / age. I’ll change that value for now - https://www.genealogics.org/getperson.php?personID=I00006058&tr... has no Birth date.
Notes
From https://www.genealogics.org/getperson.php?personID=I00006059&tr...
Sir Duncan was regarded to be one of the wealthiest barons in Scotland. His first wife Lady Marjory Stewart was a daughter of Robert Stewart, 1st Duke of Albany, and his second wife Margaret Stewart, who was a daughter of Sir John Stewart, of Blackhall an illegitimate son of Robert III, King of Scots. For his second marriage to Margaret Stewart, daughter of Sir John Stewart, of Blackhall, he obtained a papal dispensation, dated 17 January 1422/1423.
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So we have the children with wrong mother.