Patrick Buchanan, 13th Laird of Buchanan - Mother - Isabella Stewart or the NN First wife?

Looking for sources to validate the Lady Isabella Stewart as mother theory.

Wikipedia - attributes all children to the first wife (NN First wife of Walter, 12th Laird of Buchanan):
"12th – Sir Walter Buchanan,[71][72] (Guthrie Smith identifies him as 6th Chief and Buchanan of Auchmar identifies him as 13th Chief) first married an unidentified women and by her three sons (Patrick his successor, Walter and Thomas first ancestor of cadets of Gartincaber,[73] Drumikill and Carbeth) and one daughter. He married secondly to Isobel Stewart." http://en.wikipedia.org/wiki/Clan_Buchanan