Most interesting, indeed. Thanks for this very valuable information.
I have been checking data on that family for quite a while. It's not easy to follow, since your great-great-grand-father, Pierre Ducharme, married 3 times.
From his third wife, Agnes Charette, he had at least 6 children (including Ephrem, as stated in your message), that is 3 sons and 3 daughters.
Two of these daughters were baptized under the name of Rose-de-Lima, one in 1863, one in 1865. Obviously, one of them was later known as Melina. I believe that it's the older, who later married George Brodeur, while the younger married your great-grand-father Napoleon Rondeau.
If you would like a family chart for your great-great-grand-father Pierre Ducharme's family, just email me at firstname.lastname@example.org
Thanks again. Very valuable, as said earlier.
P.S.: in the first sentence of your message, you say that Melina was your grand-mother's sister. If I get things correctly, she was your great-grand-mother's sister. Otherwise, I am quite mixed up...