Why can't I press my wife's claim on Italy?

I'm currently doing a roman restoration run as Byzantine emperor. My wife has a weak claim on Italy that will not be inherited. I had the king assassinated and now the ruler is a 14 year old boy in a regency. The problem is that when I go to declare war, there's no option to press her claim.

I could press my firstborn son's claim instead but I really don't want to. He's got terrible traits and none of my electors like him. My wife just gave me a another son who's born in the purple and attractive. It would be easy to have my second son inherit both titles if I could just maneuver her onto the Italian throne.

Anybody know what to do?