Marriages, heirs etc

Had an interesting experience in my russian campaign which raised a few questions but anyway to start off heres what happened:

It's near the start of the game, and I've got little Prince Miroslav (grandson of Grand Duke Ysevelod) off conquering poland when somehow he falls head over heels in love with Princess Anotonina (Ysevelod's daughter, his aunt) and i get a message from the council of nobles to arrange their marriage, okay, much the same as the original mtw and since he didn't seem to get many proposals himself i thought why not.

anyway to cut a long story of blood, conquest and several turns more, short, Anotonina starts coming back from spain to Poland, and about half way their poor old daddy kicks the bucket. Miroslav becomes heir to the throne, and suddenly isn't eligable to be married to Anotonina even though A) the council of nobles wants me too arrange it still, and B) He isn't married.

Anyway, seemed a bit odd, she could of married any of my other non-married family members outside her immediate family (I:E not brothers). Just wondering if anyone else has had uncompletable missions like that....

Also wondering what would of happened if they had actually got married before Ysevelod died, since in MTW in-bred heirs were...a bit dim..

Oh and if your wondering what happened in the end, Miroslav married a nice polish princess, had 3 sons, who all died along with him in various battles against the Hungarians, HRE and Venetians or got the plague. Whilst Anotonina married a nice Russian noble, had 2 sons and a daughter and her grandsons and great grandsons are now ruling the empire whilst poor old miroslav's line has died out...so looks like she got lucky in the end anyway...