Hello and welcome to healthcare forum.
Antibiotic will control the infection but to reduce oedema, she will require glycerine-magnesium
sulphate dressing and foot end elevation under cover of anti inflammatory medication.
If there is any discharge from the foot then go for swab culture and sensitivity test of it and Consult your doctor with this reports, so that he can start appropriate antibiotic for your sister.
Best wishes for your sister's health.