Okay so in Seth's ending it says that he served both the new king and queen. When one sibling takes the throne, the other does not take the title of queen/king. They remain a prince/princess. However, Eirika is being referred to as the queen of Renais. This implies that she married Ephraim. You could argue that Ephraim married someone in the game, but let's assume for this example that neither have been paired off. The message is the same regardless if they have an A rank or not.
By the wording, it also seems to say that Seth served them at the same time, implying that Ephraim didn't become king before Eirika and then died so she became queen. It implies that Ephraim and Eirika were king and queen together. Now it's probably just a poor choice of wording on Treehouse's side, but still. Out of all the games to choose poor wording for that. Tana was also never stated to be queen of Frelia, as her brother is king. While it's slightly different because Innes and Tana are not twins, Ephraim is the older of the two, and rightful heir to the throne. Eirika should only be a princess, so why is she stated as queen? There's no reason to give her that title, unless she marries Ephraim or Ephraim dies. However, the game ends if Ephraim dies, so that's out.
Again, it's probably just bad wording. I just found it interesting upon reading some of the Sacred Stones endings.