She is married to James Douglas.
They got married in the year 1371, she was 31 years old.
http://trees.ancestry.com/pt/AMTCitationRedir.aspx?tid=62890030&pid=8255
Isabella Stewart, Princess of ScotlandGender: FemaleAlias name: Isabelle, Isabella, Eupheme, Isabella of MarBirth: 1348 - Dundonald, Ayrshire, ScotlandMarriage: Spouse: James Douglas, 2nd Earl of Douglas and Mar - 1371Death: Oct 26 1410 - Douglas, Ayrshire, ScotlandFather: <a>Robert Stewart</a>Mother: <a>Elizabeth Mure of Rowallan</a>Husbands: <a>James Douglas, 2nd Earl of Douglas and Mar</a>, <a&;gt;John Edmondstone</a>Child: <a>David Edmonstone of Edmonstone, Kt.</a>Siblings: Margaret Stewart, Princess of Scotland, Robert Stewart, III, Walter Stewart, Alexander Stewart, John Stuart, Lord of Innermeath & Lorn, Johanna Stewart, Katherine Stewart, Princess of Scots, Archibald Stewart, of Otter, William Stewart, Knight of Nithsdale,, Robert Stewart, 1st Duke of Albany, Elizabeth Stewart, Stewart, Marjory Stewart, Princess of Scotland