Zettai Shinanai Stella-hime

Princess Stella, the princess of the Hill Valley Kingdom, who lost her father, the king, is in the middle of a battle for succession. After receiving a will that leaves the throne and most of the king's fortune to Stella, her sister-in-law Elyusia plots to assassinate Stella. However, every attempt to assassinate Stella ends in failure. Elyusia finds this puzzling, but then she notices something strange happening to Stella's body...

(Source: Natalie, translated)