Hello...Thanks for the query.
Hair color does cause a similar discoloration in several parts of the face sometimes and it is worthwhile to switch over to safe alternatives like dyes of vegetable origin ,in case a person has to do this absolutely.
From what you mention, there is also a possibility of Melasma
in your sister.This is a chronic condition and is improved by regular use of sunscreens, avoidance if hormonal
therapies if any, use of hydroquinone
containing creams with occasional chemical peels
for face and neck.
You can discuss this with you dermatologist
and get started on the same.
Hope this is helpful.take care!