On his marriage record in Massachusetts, on familysearch, Peter said his parents were James B. Stewart and Catherine Mc(something). There are a few baptisms in the PEI Baptismal Index for a James Stewart and Catherine McNevin around the same time that Peter was born. This website suggests that James B. and Catherine went to the States (http://mackinnondescendants.com/errata-and-additions-from-home-and-from-away
). This is from an old posting online - In the Buffalo, Wy newspaper, Feb 13, 1904 is an obit for Catherine. Died- At midnight on Thursday, Feb 4, at her home in this city, Mrs. J.B. Stewart, in the sixty-sixth year of her age. The deceased, whose maiden name was Catherine McNevin, was born at Argyle, Prince Edward's Island, Canada, March 17, 1838, and lived in that city until , 189?, when she came with her husband and family to Buffalo, where she resided until her death. She was the mother of thirteen children, ten of whom survive her, three sons and seven daughters. Of the daughters, three reside here, namely, Mrs. W.S. Hill, Mrs. Charles Conway, and Mrs. B. Walker (Hellen Bernice). Mrs. Stewart was born under strong religious influences coming from a religious family, and early became connected with the Christian church. .."