Thanks for your query.
Your sister has suffered burns and still not completely recovered.
People suffering from burns may suffer contractures
due to burns.
Muscles are damaged and lose strength so it can be the reason of getting pain on movement.
Since the skin is becoming hard and the color also does not look natural there could be infection or gangrene
I would suggest you to get her to a surgeon so that he will examine and do the needful for her.
Hope this answer is helpful.