Hi, my fifteen years old sister has red spots suddenly appear on her skin - on face, belly, lower legs, inside of the lower arms and on the tips of th?

Hard to comment: Rashes have many potential causes. Those with diverse locations suggest a general issue like a food reaction or illness rather than contact with some agent like poison ivy. Without seeing the rash or having a more precise description it is hard to suggest a definite cause or treatment. If it persists for a few days or is bothersome she needs to be seen.