In the game, it specifically says that Rhagaea is looking for a man to marry Ira to reign her in and she doesn't have the support of the senators who are wishy washy about her. She is the heir-apparent, but it also makes sense that she might not inherit if her mother dies and she is unmarried (as in senators might try to pull one over on her). It is a possible scenario, would be cool if they could implement some cool stuff around this somehow. Like, another rebellion or schism or something, or she raises an army and fights back if she doesn't inherit, but if she marries she inherits. Probably asking for too much though.