When I was 2 years old, my sister was half of my age. Now Im 80 years old, how old is my sister?

Half of 2 = 1 so 1 year apart . That means 80-1 = 79 years 

if u were 2 years old n your sister was half your age then her age must be 1so her present age is 80-1=79(since she was just one year smaller than u)