SPOILER:
Thats exactly what he does
The spoiler reply is correct!
But in general it was because both the royal family and Lombardi would need heirs and spares. Since both don’t pick the oldest by default you’re going to need several kids. And then there’s her father’s territory that will need someone to take over for. So Tia would need to have… a minimum of 5 children I think. Probably more. But succession laws would probably be very convoluted on top of that.
Also the empress and head of Lombardi have very taxing roles when it comes to governance. A normal human would crumple under the amount of daily work. So a lot of historical tasks may need to be delegated elsewhere, but could be seen as “weakening the role” for both the empress and matriarch positions.
I don't quite understand. If the law forbids you from being imperial and head of the household, but you are soon to be king and queen. why not simply change the law?