Don't forget age, if your FM is past like 50, it's highly unlikely they will produce any children. I became painstackingly aware of this in my ongoing roman campaign when I married one of the daughters to a 51 year old Cnaevs Jvlivs Caesar (!!!!!), which turned out to be a child-less marriage. He had no negative fertility traits and was gnerally satisfied with everything...perhaps too satisfied, he died without an heir and my hopes for a Jvlivs Caesar-family branch was snuffed out...