The idea comes from Visconti, who has been the one stuck riding herd on his bastard daughter the majority of the time. (He's the only one strong enough to do so, and with hindsight, it had probably contributed to his Cloud suggesting this as a solution.) It makes a lot of sense; they needed the Cavallone to keep guarding their flank, and Xana was of an age with their remaining heir.

“Most of the time, curses aren’t specifically or intentionally cast,” Alex explains. “It takes a lot of supplies, and time, and energy to set a curse. Most magic carriers won’t do it; it’s not worth it.” “If it’s not a curse…” “It’s a mutated spell.”