The Empress has already held a grudge against her, likely fearing that in the future she would monopolize Saknu's favor, and use it to act against her. Therefore, she carefully picked these two little demons to share the favor with her. If these two beauties could win favor in the future, they could contribute to their grand plan. Even if they weren't able to curry favor, at least, they could serve to infuriate her, thus not wasting all of their machinations!
Bai Shang Palace saw through Empress's scheming plot because of Cai Wei, her face burning up a little but still holding a forceful grin, she said: "The Princess and the Great Prince are essentially one, if they serve the Princess then they are serving the Great Prince, and if they serve the Great Prince then they're serving the Princess!"