
It's been a while since I read it but I believe the stepmom's doing it bc the she is the crown princess bc her mother was empress. While her son is a prince but she's only a concubine. So there's a difference in ranking. With the princess out the way her family can take control and instate the prince as the crown Prince. There are multiple parties who want the crown princess on their side bc who ever she marries can become in power and be the next emperor( hence why they want to kill the prince). On the flip side The evil stepmom wants her child to be the next ruler& have her family be in control. To do that she has to get rid of the princess. I'm certain it's something like that
I've only read the prologue, but why would the step mother want to kill the daughter? It's not like she can become the next ruler because she's a girl, virtually harmelss. Oh well, time to find out.