A good point of reference is Dr. David Faux's book "Understanding Ontario's First Nations Genealogical Records". In his book Dr. Faux specifically references the Mary Lancaster file and he explains how the claim that she and Jacob Brant were married and had issue is false.
http://globalgenealogy.com/countries/canada/first-nations/re...His book is available at most libraries or Amazon.com
Rev. J. Files and his wife Angela of Brantford, Ont. have constructed a comprehensive genealogical record of the Brant/Croghan tree. Previous mention of Mary Lancaster has been removed from their research too.
Sondra Brant