His allegiance honestly may not matter much, and it’s not like House Tarth has been sending a lot of resources to back up any particular claim to the Iron Throne since Renly.
Brienne is undeniably an ally of the Starks, at least one Lannister, and Daenerys at this point, but Brienne is technically not the head of House Tarth.
She may not survive to the end of The Last War, and she’s Selwyn’s only child.