When “House of the Dragon” premieres on HBO later this month, it marks a return to all the familiar tenets of Westeros made famous by “Game of Thrones”: Sex, violence, and, of course, dragons. But the prequel series based on George R. R. Martin’s “Fire & Blood” centers something different from “Thrones”: A pivotal relationship between two women. “House of the Dragon” is about the Targaryen family, war in the seven kingdoms, and the Iron Throne itself, but it is in a very real sense it’s about Alicent Hightower (played in early episodes by Emily Carey and as an adult by Olivia Cooke) and Rhaenyra Targaryen (Milly Alcock; Emma D’Arcy).