Sorry if this is out of place, but this error has been bothering me for a while now! On the House Branfield wiki page someone has put that the house is originally from the Crownlands without giving a source. I don't remember any dialogue from ttgot that states this, and over on the official website both Lady Forrester and Malcolm's descriptions only state it was a Southern house.
Seeing as Mira Forrester is written as a Tyrell handmaiden, I would hope the Branfields would be from The Reach (lest her occupation be completely nonsensical), but without a description they should just be "Southern".