Though in the book, Idgie and Ninny are two separate characters, at the end of the film it's largely hinted that they are one and the same , although this runs counter to Ninny's earlier comment that ''I was practically adopted by the Threadgoodes; I married her (Idgie's) brother, Cleo''.

Can we conclude that was ninny threadgoode really idgie in fried green tomatoes?
no