Eikoku Monogatari Shunka Torikae ShouEikoku Monogatari - Shunka torikae Shou; 榮国物語 - 春華とりかえ抄

The twins, Sei Shunran and Sei Shunrai, are often mistaken for the wrong gender. The older sister, Shunran, is smart and the younger brother, Shunrai, is beautiful. Shunran wants to take the civil service exam and not enter the inner palace. So the twins switch places. Shunran will take the exam, and Shunrai will enter the inner palace.