Well Duke John's the dude you want to talk to for that... though BI's not really the most perfect game engine for this - MTW really is more appropriate. BI lacks the marriage alliances that really make the Wars of the Roses period work (and a lot of other things too).

Been kicking it about in my head forever but without the marriages (and to a lesser extent the Crusade-Jihad function) it's just not going to work except for eye candy.

Best of luck anyways,
Harlechman