Chou!!! Tensai Hatsumei Reijou no Powerful Ryouchi Kaikaku

Marianne, the second daughter of the Count of Landfuria, is a book-loving, child prodigy. Her current worry is that she lacks the refined grace expected of a young lady, which makes her unpopular with her elegant older sister... But Marianne is always optimistic, and her cheerful personality gradually melts her sister's stubborn heart. What's more, her innocent and carefree nature seems to have caught the attention of the second prince as well—!?

(Source: Hobby Japan, translated)