The Cursed Face on My Skin

(from ebookjapan):
When 12-year-old Yumi invites her classmates to her birthday party, her prankster sister Mari puts a toy toad's leg in her food. The dish is then eaten by Sayuri, who hates toads to an unusual degree. When Yumi goes to visit Sayuri, who has taken the day off from school, she is attacked by Sayuri who has turned into a toad. ......?