There is an entry in Family search for Jean Poirier (of France) that says he is. "listed as the brother-in-law of GUILLAUME BLANCHARD (Janet Jehn, Acadian Descendants, V. 1., p. 16)." Can anyone do a look up and see if that's true and if there is any further information? The Guillaume Blanchard (Blanchard-173) in the family is married to his wife's daughter by her second husband (after Jean died), this doesn't seem too likely.
