She is probably stressed/sad about loosing her sister. Give her appetizing (to a chicken) food - like mealworms, scrambled eggs, or oatmeal. She might get over it, she might not. Does she have any other chicken buddies? If she doesn't, get her one ASAP! Chickens are flock animals and even if you spend 24 hours a day 7 days a week with her, she is going to need a chicken friend.