I'm like 99% sure that this is actually way more based in "historical fiction" than actual "history," because from the perspective of actual, historical inheritance law, someone would be getting scammed in that deal. I'm just not quite sure who. One way or the other someone is getting fooled into trading something for basically nothing of actual value to either side, because harem concubines basically had no legal standing and were closer to sex slaves (often literally purchased for money) than to actual, legal wives whose children would be in the line of succession. So neither Pimp Daddy nor Emperor Wannabe would actually get anything out of it besides another sex toy, because her children would be able to inherit neither the throne nor their grandfather's properties and titles.