Using the age in the 1871 census, your Olive is probably the following person born and baptized at Arichat, NS as "Adele" in 1844

Desroches, Adèle
baptism date: 1844-09-18 birth date: 1844-09-17
father: Georges Desroches
mother: Olive Forêt
godfather: Edmond Terrio
godmother: Angélique Forêt

Since I have not seen the exact register entry, I cannot say for sure if Adele is a correct transcription of the name, but your "Olive" would definitely be a member of the family of George Desroches and Olive Forest, who married at Arichat, NS. Unfortunately, since the earliest records were burned in November 1838 (rectory fire), their marriage is not available. Perhaps you can find out connecting info at the various Desroches and Forest family message boards or websites. Both names are Acadian.

Dennis