"If the bastard makes his first wife the crown princess, no other powerful families would want their daughters to be a concubine."
The first wife was considered the lawful wife. And women who married afterwards were placed in a lower position. If the crown Prince wanted other wives, the difference in status between the first wife and the others was huge. The more powerful the elites was, the more likely he wouldn't want his daughter to be in a lower position.
Furthermore, to make an alliance with Hamon meant to turn their backs on Merona. Nobody would like to cross her, and no family would allow their daughters become a concubine. As long as she lived, she'd prevent him from getting any alliance with any powerful families.
"Wow! You are so smart, Sister. But I still think there shouldn't be a crown princess to begin with."
