Ishinana

A girl who has lost everything sets a brutal trap for revenge on the royal family that rules a great nation. A contract has been passed down in the 300-year-old Seiin Kingdom. For generations, it has been a contract to welcome a shrine maiden from the Kagemori People, an eastern ethnic minority with unique powers, as empress. However, six months after the shrine maiden Somali, who was in love with the new emperor, Eigaku, was welcomed as empress, the contract was broken in the worst possible way. Her younger sister, Kukuna, infiltrates the harem as a palace maid to avenge her sister... Source: Comic Walker, translated