So this one isn't so hard as to be impossible. It will just take patience and luck.
As detailed in the above video you want to pick the 867 start and choose to play as Marzoban Wahsudan of Gilan
. Reason being his religion allows familial marriage.
Follow what Enigmaticrose4 does in the video and with a bit of luck you will get the achievement in a couple of hours.
Just remember you must be in Ironman Mode for achievements to unlock.