[quote=@Xilaw]
[quote=@LordZell]
If we get a marriage and we marry the eldest daughter then we'd kill all the trueborn sons till she would become the heir and wait for the Duke to die.
[/quote]

Sounds easy enough. Unlucky for you, there's no single women in my family at the moment and those born in the main branch are most likely going to be wed to monarchs of other countries ;)
[/quote]

Start an affair and throw yourself at their feet quick that way a foreign lord invades and starts beef!