I was recently told that the provinces of Elevesmiere, Nabhriene, and Tuor's Hold were originally part of Avanil since Imperial times untill as recently as a few hundred years ago.

If so then how did they become a part of Alamie/Tuornen?

I was told to check out the Tuornen sourcebook, but alas I am without. Would someone be so kind as to relay the gist of what the publication says about Avanil.