Evidently someone doesn't read the lore? All right then, here's the quick run down. Prince Uldren's plane was severely damaged. He crashed landed on a nearby planet. Which honestly makes no sense, since the only planet nearby is Saturn, which is a gas giant, but that is what the lore says. He survived the landing. From there, he attempts to use his awoken magic to sense his sister to determine if she is still alive. He is able to sense her for the briefest moment (indicating that she is alive) before he is distracted and loses his concentration. It is also known that the voice actress for Queen Mara still actively works at Bungie. So she definitely will come back in the future, and because of the grimoire with the brother, I suspect she will be very much alive. But if not, she will be there as a flashback or ghost of some sort. But in regards to your question. Prince Uldren is in charge of the reef until the Queens return. However, he shares the Throne with Petra, because the Awoken are the kind of society that believe women are more fit to lead than men. This is all assuming that he used his Crows to return to the reef, of course. If he didn't, and is elsewhere, Petra is the sole regent. Edit: the grimoire card that explains what is going on with the brother is Allies : The Queen : Aftermath. #TitanMasterRace