Thank you very much in trying to help me solve this puzzle.
In my original post, I mentionned Alfred Leger alias Allen and Isabella Cormier were granted a dispense to the 4th degree for cause of blood relation. I'm not sure of this is properly said in english, but in French, it is said: "Dispense de 4e degré pour cause de consanguinité".
If I make the following assumptions, I think that I can also explain the dispense:
Alfred is the son of Anicet Leger and Philomene Bourque Isabella Cormier is the daughter of Francois Cormier and Domithilde Babineau
If the above assumptions are correct, then Alfred (Alphée) is the great-great grand son of David Melanson and Marguerite LeBlanc. Isabella Cormier is also, the great-great grand daughter of David Melanson and Marguerite LeBlanc. This would explain why they were given a dispense to the 4th degree, and also that Alfred is the lawful son of Anicet Leger and Philomene Bourque.
I don't believe that Alfred was adopted. But I am still wondering as to why he would change is name to Allen. I know of another case where a Leger changed is last name, Peter Leger alias Peter Ryan, son of Gervais Léger and Céleste Breau.
According to my data, Anicet Léger and Philomène Bourque have four children:
Philippe Léger, born January 4, 1865, died Jun 18, 1866 Alphée (Alfred) Léger alias Allen, born November 17, 1866 and married to Isabelle Cormier, daughter of Francois Cormier and Domithilde Babineau, October 23, 1895 in Amherst, NS. Céline Léger, born August 9, 1869 Céline Léger, born June 9, 1870 and married to Marc Boudreau, son of Hilaire Boudreau and Cresence Bourque, November 27, 1893 in Sackville, NB.
Anicet is the son of Hyppolite Léger and Osithe Boudreau.
There is one possibility that I haven't explored. Alfred's father died in February 4, 1872, when Alphée was 5 years old. Did Philomene, Alfred's mother, remarry? If so, could it be that she remmarried with someone who's last name is Allen. I don't have any information about Philomene after Anicet died.