Dear Distraught,
Your sister may have a tumor or other condition, though it may not be so serious. There is a show called medical mysteries which deal with such conditions which are not diagnosed or are done incorrectly.
Please do not accept even the hospital's word and assume that the condition will just cure itslef if your sister is in pain, or think that your sister's condition is due to her overeating0. Find the doctor which you and your sister feel is truly helpful.
( The show once even displayed a case in which a bright college student stated that she was experiencing such depression and pain that she could often not get out of bed. The doctors which she originally went to, could find nothing wrong, and she was though to be experiencing a psychosomatic condition brought on by issues of independence. However, the show found that the young lady actually had LYME disease, started her on medication, and helped her so much that she even completed her education!).
Lots of luck, please do not give up, or become immobilized ( which I sometimes feel like giving into).
Sincerely,
Barfe