The Holy Britannian Empire is ruled by the Britannian Imperial Family, or more formally, The Imperial House of the Holy Britannian Empire. Its members often hold high positions in the government and/or military. They are descended from Richard von Britannia and Queen Elizabeth III of Great Britain and Ireland.
Each member of the Britannian Imperial Family is ranked according to their birth order and order of inheritance. This is also separated by gender. For example, Odysseus eu Britannia is the 1st Prince of Britannia and nominally heir to the throne. Guinevere de Britannia is the 1st Princess and 84th in line for the throne.
The Sovereign has many rules that apply to only them. However, there are general rules that govern the rest of the Imperial Family. Unless given explicit permission by the Sovereign, each male member may only take one wife that must be approved by the Sovereign ahead of time. In a way, the Imperial Family acts like one giant noble family, with the Sovereign acting as the Patriarch or Matriarch of the House. But in many others, it does not.
Emperors with multiple wives have additional rules. All but 2 Emperors of the modern empire (Richard and Arthur) have taken multiple wives in order to gain more and more children. Each member of the Imperial Family will have the same nobility particle as their mother. This is so that one can tell which consort a Prince or Princess belongs to.
An Emperor can take as many consorts as they wish to. While in theory an Empress could take on multiple husbands, none of the previous Empresses have done so. After marrying a consort, they form a strange cadet branch between the royal family and whatever noble family that consort belonged to. Their name remains the same as it was, but their regal surname is added on at the end. For example, Consort Eudokia li Britannia's full name is Eudokia Antonia Winifry li Britannia, at birth her name was Eudokia Antonia Winifry.
Every firstborn son by a consort is granted a barony for their family line. Unfortunately, this means for a Princess, unless they aquire a title of their own, they are basically marriage fodder for alliances. Each line of the Imperial House acts like a noble house of their own, just tied much closer to the Emperor than the other nobility of the Empire. An Imperial Consort is head of her own line, and that descends by order of male-preference Primogeniture.
Exactly what constitutes the Imperial Family is up for debate. Officially according the Britannian Constitution, The Imperial Family constitutes the Sovereign, their spouses and children, and if they choose, their siblings and niblings. Beyond that, no one else is considered part of the Imperial Family. Princesses who marry out of the Imperial Family officially no longer are part of the Imperial Family, though as a courtesy they are still addressed as Princess.