Futago no Ouji ni Futago de Konyaku Shita kedo "Ja nai Hou" dakara Yami Mahou wo Kiwameru

Shuri, a demi-beast of the Cat Tribe and a prince of Ryupen, was always being compared to his twin brother, Luca. No matter what was compared—looks, fur color, intelligence, or magical ability—Luca was always superior. Coupled with the superstitions surrounding unlucky black cats, Shuri was constantly looked down upon by those around him as 'the other one.'

One day, he meets the twin princes of the neighboring country of Lindenberg: Siegfried and Gilbert. Having become a fiancé candidate for the older twin, Siegfried, Shuri begins to study desperately in order to help make him king. However, no matter how hard he tries, the gap between him and his genius brother only continues to widen...

Source : https://www.alphapolis.co.jp/manga/official/627000760