I believe I know who Joseph Norman(d) was and Mary Seguin was likely his wife Marie Jeanne-Chantale Seguin. The following note I received from a fellow Seguin researcher some time back may explain why Joseph and Mary had the opportunity to attend your ancestors' wedding. This is the note I have on Joseph Norman:
Joseph Normand has been extremely hard to locate. He was a day laborer at farms and spent much of his time moving his family around in search of work. He was enumerated in the 1860 Madrid, St. Lawrence Co., New York census but subsequently left New York. His family left Madrid, New York prior to 1865 because they were enumerated in the 1865 Madrid, St. Lawrence Co., New York census. It is my belief that the family left New York prior to the start of the Civil War because they felt it was not their fight, nor their country to be involved in the fight. Joseph Normand and his family were enumerated in the 1871 St. Anicet, Huntingdon Co., Québec census. I believe that Joseph Normand and Chantal Sequin stayed in the St. Anicet area until their deaths. Chantal Sequin died in 1875 and her burial record was located in the Ste. Agnes de Dundee parish registers in Huntingdon Co., Québec.
It is possible that Joseph was a neighbor or an employee or a fellow worker of your gg grandparents?
Sorry I have no further information on you Abraham or Sarah.