Okay, a search of the text of "The Godfather Returns," by Winegardner, includes a Cast of Characters that identifies Mama as Carmela. It looks like the book was published in 2004, and searching on here it looks like no one used the name until 2005.

I've never read any of the sequel or prequels, but it looks like that could be the source.

Edit: very strange they'd pick the same name as Mrs. Soprano, if the Winegardner book is, indeed, the source.