Can you tell me what the phrase used by members of the British royal family for "Their Majesty" on the phrase "The spirits of the Emperors of the Empire of Sriwijaya and the spirits of descendants of Their Majesties", because the term "Their Majesties" is

In the context you've described, when referring to the spirits of deceased monarchs, members of the British royal family (and other royal families as well) might use the phrase "Their Late Majesties" to indicate respect for those who have passed away. This expression acknowledges the former status of the monarchs while clarifying that they are now deceased. Therefore, you could say: "the spirits of the Emperors of the Empire of Sriwijaya and the spirits of the descendants of Their Late Majesties."