Shirobuta Kizoku desu ga Zense no Kioku ga Haeta node Hiyoko na Otouto Sodatemasu @COMIC

An empire falling to ruin: The Kio Empire. Living in that empire, is an overweight 5-year-old abandoned by his parents, Prince Ageha. Ageha has memories from a previous life of him living as a Japanese man in another world. He then encounters his younger half-brother, Regulus. Ageha had a vision that Regulus will kill him in the future over inheritance, so Ageha sets out to raise him after Regulus's mother passed. Once the sparkling and fluffy-haired Regulus cozied up to Ageha, He turned into a doting older brother before he even realized! Enjoy a fantasy story of young brothers managing their kingdom, sometimes involving the gods, as they reform it step by step to bring a better land to the next generation! (Source: MangaPlaza)