The Splendid Territorial Reform of the Exiled Baron's Son

The protagonist Ryu, born into the Ardrén baronial family, has been unable to use magic since childhood due to “a certain reason.” Branded as incompetent by his father, he is cast out to a separate residence along with his mother and his younger sister, Lena. Although they live modestly, they manage to find happiness—until their mother falls seriously ill. Upon learning this, the father—who has always seen them as a nuisance—plots to have all three of them killed. After discovering the plan, Ryu makes a decision: to defeat his father in order to protect his mother and sister, and to develop the territory he has obtained into the very best it can be, so he can afford his mother’s medical treatment. Thus begins a heroic fantasy of territorial management starring a noble scorned as useless—who turns out to be the strongest of all!

https://www.cmoa.jp/title/364979/