I tried that, but it's difficult to kill a duchess in a foreign court with another culture and religion. At least my dynasty ended up with it.\
Actually, that particular son is a funny story. He was a twin, so I gave one to my wife to raise and took the other one. That one came out amazing - 17 Martial, 18 Stewardship, above 10 in everything else, and good traits. The one I raised had crap stats, became gay, and then died in the first battle of a war that happened.