Hmm, I'm not too sure, yet I think that this, may be, a deliberate feature of the game. In the manual, and on the section about princesses, it, most peculiarly, says:
"if you drop a princess on one of her brother's, or her father, then she will be removed from the game with some nasty consequences"

Perhaps it was placed in the game, that is having royal heirs having children, but not being married, to add a bit of flavour and make you wonder about how respectful and regal your "Imperial" family really is! In many cases, probably not at all - especially if they have vices such a "Perversion" or "Adultery" too!