I totally agree. Adoptions were common place throughout the history of interaction between Europeans and Native Americans. DNA is of no consequence in this kind of situation. It is obvious Sanscrainte lived as a Native American and probably thought of himself as one.
Because Europeans, the French, in particular, were in Michigan for over two hundred years by Sanscrainte's time, `heritage' was often decided by the person's choice. I would look at the familial relationships as well as the interactions within the community rather than the DNA.