Thanks for the query posted .
I would be pleased to answer your question .
Your condition and your mother's condition of skin darkening are totally different .
Your condition on the knee can be due to friction or hormonal
changes or Vitamin B 12 deficiency .
You should get Chemical Glycolic peels done for the knee to lighten it or even have multivitamin
Avoid friction on the area .
For your mother's condition :
She should apply over the counter Vitamin C
serum on the area twice a day daily
Thanks and take care