Quote Originally Posted by Weebeast
How would you feel if you married my sister and I married yours?

Jokes aside, non-ally factions seem okay sharing their princesses with me.
Hell, nothing that uncommon, actually My brother and I are married to two ladies who happen to be sisters

Jokes aside, it is true that minor factions (Sicilians, Danes and also Aragonese) seem to be real princess-spammers. Of course, they also have their host of (mostly) unmarried princes, so there you go, trade your royal kin with them all the way you like