On the Silent Sisters, isn’t there a cultural and historical vacuum in the prestige and power of septas-septons? Even in a military minded Westeros, sending extra offspring to religious institutions where they could rise high, hold office and land ( like Princess Sophia Albertina).

Well, we do see some highborn septons and septas. The Hightower High Septon who started the Revolt of the Faithful, Luceon Frey is one of the Most Devout, Septon Sefton from Sworn Sword is highborne, and then of course there’s Baelor I who became a septon, and his sister Rhaena who became a septa.