How to mod inheritance claims?

I'm playing vanilla now and the great houses dying like flies. So want to ask how do I make a mod like the one in AGOT? Where descendant can claim their ancestral land/county if not hold by their dynasty? 

Tried looking into the AGOT files but I just couldn't figure it out what/which to copy? Just for my own use of course, not publushing it outside.

Please?