Hi, welcome to HCM.
As you have mentioned your sister's problem with itchy lesion over the back and a pus-filled lesion means she is having an infection now. At that moment, steroid will be problematic. Probably she is having some generalized eczematous condition that may also be Ichthyosis
or contact dermatitis
. So it needs a proper diagnosis & then actual therapy. Nowadays effective remedies are available. Don't worry. Kindly consult a good dermatologist
. He will examine her & if needed will do some investigations.
Hope she will be ok.