Hi.
Thanks for your query.
Noted the history about your stepdaughter and understood the concerns.
Fever coming after 10 day of appendix removal has to be taken seriously unless proved otherwise.
The possible causes are:
Infection in the abdominal wall due to
bacterial growth in the incision.
Infection deep within like you said
peritonitis as evidenced by pain in abdomen fever and feeling sick.
Other reasons like flu or other infection like
urinary tract,
pneumonia should also be considered.
I would advise your the following in such a situation:
Since she is admitted Doctors sure will get the investigations and clinical evaluation done.
Blood culture and other tests like CBC.
Urine routine, micro, culture and sensitivity.
CT scan of the abdomen.
Course of broad-spectrum antibiotics should be given.
Keep her in hospital till the proper diagnosis and proper management is done.