I'll Live a Long Life to Dote on My Favorite Stepbrother!

Saioshi no Gikei wo Mederu Tame, Naga...

Alba is reborn as the son of a poor nobleman. He is unable to use magic due to a disease and is destined to have a short life, so he is taken in by the duke's family. There, he meets Orsis, who was his favorite person in a previous life. While this coincidence seems to good to be true, he remembers that he is why his beloved lost his smile, and begins to search for a way to live a long life so that he can continue to love him. (Source: MU, edited)