Queen Elizabeth II has been the British monarch since 1952. As she's now in her 90s, the line of succession for the House of Windsor will kick in sooner rather than later. Fortunately, no one has to worry about a real-life Game of Thrones, with fewer dragons and more tea and crumpets, taking place, as the process has strict rules dictated by history and Parliament.
However, the unexpected does occur, even in a royal family. Elizabeth's uncle, Edward VIII, was supposed to marry and have children of his own, but he abdicated to marry American divorcée Wallis Simpson and thus altered Elizabeth's destiny. So while those further down the line of succession are unlikely to sit on the throne, they're expected to be ready, just in case. In the meantime, some now have to hold outside careers.
The monarchy has also demonstrated it can (occasionally) change with the times. The Succession to the Crown Act 2013 kicked the centuries-old tradition of male primogeniture to the curb. This means younger male siblings no longer take precedence over their older sisters — though the change only affected royalty born after October 28, 2011.
No matter what may change in the future, and whoever takes the throne, members of the royal family — those in "The Firm," as they often refer to themselves — are used to following the dictates of duty and tradition to fulfill their obligations. Those who succeed Elizabeth — like Princes Charles, William and George — will no doubt rely on these watchwords as they guide the monarchy and the royal family into the future.