I know that the primary audience is aimed at girls/young women, but you know what? He could just take her Marie as a concubine. Marry Lady Rachel, have her be empress, and then take Marie as his concubine. Or, if that's not allowed since she's a war prisoner, she is still literally his property. He owns her body. He can do whatever he wants with her.
That's so much more realistic than what is likely going to happen.