The name Ismeria, very probably a feminine variant of the Germanic Ismar, appears linked with the worship to Our Lady of Liesse. According to the legend, Ismeria was a Morish girl who was converted to the Christianism and released the crusaders captivated by her father because of the apparitions of Mary.
The devotion to Our Lady of Liesse was not only important in France, since the 12th c., but also in the general Medieval Christedom. That is why it is not surprising that the name Ismeria was taken up again in The Golden Legend (c. 1260), by Jacobus de Voragine. In this work, Ismeria is the sister of saint Anne, Mary's grandmother.
The popularity of The Golden Legend all across Europe spreaded the name, which appeared in some later works, as José de Valdivieso's plays in Spanish (16th c.-17th c.); since in Voragine's work the name is only cited once (Sept 8th) and in passing, it is not surprising the limited use outside of France.
The name Ismérie has been used in France more or less regularly since the Middle Ages, sometimes spelled Ismeria because the documents were in Latin or at some times because Classical and Italian names ending in -a were a trend, just as currently and in the 19th c. That is showed, for example, in a passage of Madame Bovary (1857).
Since the OP's great-aunt was French Canadian, the use of Ismérie/Ismeria is not surprising, because that has been a kwnon name in French since the Middle Ages. In fact, Ismérie, with variants and derivated forms, was a common name in Québec in the 19th c. (http://sites.rapidus.net/jhuriaux/femmesi.html lists Isméria, Ismérianna, Ismérianne, Ismérie, Ismérienne, Ismériette, Ismérina and Ismérine).