You can 
put this solution on YOUR website! 
This should be easy enough to solve in your head.
From "When Vanessa was 4 years old, her sister was half her age. "   you get
that her sister was 4/2 = 2 years old.
The difference between their ages (which is 4 - 2 = 2) stays constant for their entire lives, so when Vanessa is 100, her sister is 100 - 2 = 98.
Hope this makes sense.