Looks like this skin rash
is some kind of allergic rash
.Maybe he is allergic to some kind of food,dust mites,pollens etc as it is recurring.Since her sister does not suffer from the rash that means it is not a communicable infection like scabies
.You could apply calamine
lotion for soothing effect;but get him examined by his doc because if it is allergic he might require some creams and medication to treat it.