To put her royal beloved on the throne, a young and ambitious mistress has to plot against his elder and much more charming brother.

Sounds interesting, but it's a book, not a movie.

http://www.allreaders.com/topics/info_15054.asp

Apparently she's trained into a ninja...

You might be interested in the book Assassins of Tamurin.
http://www.amazon.com/The-Assassins-Tamurin-S-Tower/dp/0380806215/ref=cm_cr_pr_product_top
An young orphan is groomed by a vengeful mother of an orphanage into hating the current ruler. Sent to spy on him and undermine him, she falls in love him.

"When an ambitious concubine plots to supplant the rightful heir to the throne with her lover, she realizes her pray is her one and only love."