Thank you for posting your query.
i will like to know the size, shape, elevation above the skin level, consistency etc (all the possible characteristics that you could identify and describe). It is possible that your sister is suffering from hyperpigmentation
due increased melanin
production, side effect of drugs, underlying diseases condition such as chronic liver diseases, diabetes
etc We could also have melanoma
( a skin malignancy), acne, freckle etc.
You will to take your sister to a dermatologist
who would take a closer look at the skin lesion and do a proper evaluation before treatment could be established. If dermatologist not available then you will to consult with a doctor with experience in skin condition. A photo the spot would have facilitated our discussion. I will love to hear back from you with further info on the appearance of the lesion so we could determine what lesion is presenting.
At 21 years she might most likely be having acne or melanoma except an underlying chronic diseases is present.
Hope this helps