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Your sister seems to have Atopic dermatitis
which includes dry skin
scaling and allergic tendencies
is commonly seen with dairy products like egg milk, wheat,sea foods and nuts. If she is allergic to these she has to avoid it otherwise it may cause her harm in the form of allergic rash
to even severe anaphylactic reaction
As for milk u can try soy formulas and rice and pulses and fruits may be used .
As for dry skin she has to use emollients or moisturizers daily n mild glycerine soap only.
For allergic reactions pls take prompt treatment from doctor.
With age the symptoms do decrease
Kindly consult a dermatologist
for better results